My airconditioner is blowing hot air. The mechanic said that the compressor is not broken. He needed to find the lower side pressure valve to kick the compressor on so it would stay on. Where is the valve?

A mechanic would check the system with a set of pressure gauges. If the compressor is running, the low side would read around 30psi and the high side would be around 250psi. That is how you know if the system is full of freon or not.
You may have to get a professional diagnosis to proceed. If you replace the compressor, the accumulator needs to be replaced and a vac pump is used to pull all of the air out before the freon is added.

see this causes and fix it. God bless you
Compressor Checks
Start with the compressor. Does it engage when you turn on the A/C?
If so, the compressor is working and the A/C system probably contains enough refrigerant to make cold air, so the problem is inside the HVAC unit. Replace the motor that controls the blend air door (this is a difficult job and best left to a professional since it involves tearing apart the HVAC unit -- about an 8 to 10 hour job!).
If the compressor does not engage when you turn on the A/C, see if it will run by jumping the compressor clutch wire directly to the battery (use a fused jumper wire). If the compressor works when you jump it, and the A/C blows cold air, the system contains refrigerant and the fault is likely a bad A/C compressor clutch relay or a bad clutch cycling switch or pressure switch.
If the compressor does not engage when you jump it, the problem is a bad compressor clutch.
If the clutch engages but the compressor does not turn (the belt will start to slip and squeal), the compressor is locked up and you need a new compressor.
If the compressor clutch engages and turns the compressor, but the A/C still does not blow cold air, the system is probably low on refrigerant and needs to be recharged.
Refrigerant Checks
Connect an A/C pressure gauge to the HIGH SIDE service port (located in the high pressure hose that runs between the compressor and the condenser in the front of the engine compartment). The gauge will tell you if there is any pressure in the system. Simply depressing the service fitting valve with a small screwdriver to see if any refrigerant squirts out is NOT an accurate check because it tell you how much pressure is in the system. It may still have some pressure but not enough to trip the low pressure safety switch so the compressor will engage.
If your A/C system is low or out of refrigerant, check for leaks, then have the A/C system vacuum purged to remove air. After the air is out, it can be recharged with the specified amount of refrigerant. It is important to get any air out as this will reduce cooling efficiency and may make the compressor noisy.
A/C System Functional Checks
If the refrigeration circuit seems to be working (refrigerant in the system, compressor running and building pressure), but there is still no cooling, the problem might be an obstruction in the orifice tube (located in the high pressure hose between the condenser in the front of the radiator, and the evaporator located in the passenger compartment). A blockage here will prevent the refrigerant from entering the evaporator or recirculating through the refrigeration circuit.
If the orifice tube is plugged, the high side pressure reading will be lower than normal, and the low side reading will also be lower than normal because no refrigerant is circulating through the system.
If the refrigeration circuit seems to be functioning normally (compressor running, frost or condensation on the high pressure line from the condenser to the evaporator), but no cool air is blowing out of the ducts inside the car (and the blower is working), the fault is likely a BLEND AIR door that is stuck in the HEAT position, or possibly a badly clogged cabin air filter that is restricting airflow. Another possibility would be a fault in the automatic climate control system such as a ad interior temperature sensor or control module.

Low side pressure should be 28-32 PSI, you have a big problem here, possible expansion valve issue.

Hi. There seems to be a blockage within the system. I will list all parts to be checked by your mechanical. I would also have the blend door linkages or servos inspected for fail points along with the listed areas of concern, below:

Loose or broken drive belt
Inoperative compressor or slipping compressor clutch
Defective expansion valve
Clogged expansion valve, receiver-drier or liquid refrigerant line
Blown fuse
Leaking component: any of the parts listed above or one of the A/C lines, hoses or seals.

Instruct your mechanic to check all the above, along with the dash blend door linkages for faults.

There could be a blockage in a screen called an orifice tube in the liquid refrigerant line. The refrigerant may not be circulating properly and this can cause a pressure out of range (too high) which can kick off the compressor until the pressure is no longer out of range. Another possiblity is the compressor is nearing the end of its life and gets too "hot" and kicks off. I have seen some that would "smoke" from the clutch failing before any thermal limits would kick off the compressor, so I sort of doubt that's the issue. Another possiblity is a loose drive belt, but that would ordinarily present a belt noise (loud squeal). Another possiblity is the alternator is failing and cannot deliver sufficient current to run the electrical devices that kick on when the AC is running. A parts store can check that for you for free!

Above solution is correct, had the same problem with mine and it was the expansion valve.

If it is above 100PSI, you are over charged. Any thing under, should be normal to low. You probably need to get it checked by a professional to make sure it has the correct charge, and the compressor is getting power when the switch is turned on.

/Does the compressor kick in compressor or the compressor clutch might be shot

Could be the expansion valve. That's what allows the temperature to actually drop in the system - the contents of the AC system are under pressure, and are at a certain temperature. When the freon goes through the expansion valve, the pressure drops suddenly and dramatically, which "flash chills" the freon, allowing the air to blow cold. If it fails, you may get lukewarm air from the AC despite everything else working. Also though, check to see whether the AC evaporator fan (underhood) is working - on some cars, if it doesn't turn on, the compressor won't be run enough to cool the air.

Check to see if your system is charged. there is a pressure plug with a green cap near the right side motor mount. If you have pressure then check fuze under hood and see if compressor is kicking in. If not charged it will not kick in.