I am attempting to install a dishwasher drain line to a kitchenaid disposal under my sink. The disposal is a Kadette 250 1/2 hp. There is some kind of block or plug stoping the water from entering the disposal. How do I unblock it.

sounds like the pump is gone, or a blockage.

If you hooked up the drain to the disposal directly there is no vent in the line. Use the air vent on the sink to connect the dishwasher drain hose. If there is no air vent, then you'll have to install one in the sink.

The dishwasher usually drains into disposal, or directly into tailpiece, and water drains down to the p-trap.
Trap may be clogged or stopped up.
Clean out drain line, tighten connection where dishwasher attaches.
Clean out bottom of disposal under cutter blade.
Copy following link for clogged disposal, and drain line tips.
http://waterheatertimer.org/How-to-repair-and-install-Garbage-disposal.html

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The dishwasher's old water is pumped into the same drain that your kitchen sink operates off of. i would recommend that you check the hose that runs out of the dishwasher to the pipes under the sink to make sure that the line is, a) hooked up, b) that it is not crimped and causing the water not to flow.

If the leak is coming from the bottom of the disposal check the sink connection, drain connection, and housing gasket. If you find these are all good and you can see the water is definitely coming from the disposal itself and not from any of these connections, then the internal seal in the disposal is bad and will need to be replaced. If the disposal is over a few years old then it is more than likely best to just replace the unit. Not cost effective to have it repaired.

If this is an old installation and the unit has been working then the problem is with your house-sink drain.
Dishwashers either connect to your disposal unit or to the drain directly. Usually if you have a clog with a disposal unit in the line then it backs up into the sink through the main drain. You say from the overflow.
Look under the sink and find the point where your dishwasher connects to the main drain. Open the connection and check for a clog just below the connection point. This is the most likely cause of your problem.
Good luck.
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Try turning off the circuit breaker and turning it back on, pushing the reset button firmly on the bottom of the disposer, and checking to make sure it's not jammed and rotates freely. If it is jammed you can put a disposer wrench or allen wrench in the hole in the bottom and rotate the shaft to free it. There might already be a wrench under your sink for that.

Hi, You need to punch out the plug from the outside. The plug will fall into the disposal and you can reach in to remove it.
I use a small screw driver and punch through the plug in a few spots to weaken the plug. This will allow the plug to release easier.
Please let me know if this helps you at all or if I can assist you further.

Thanks
Vic

Three points to ck. !) make sure disposer is plugged in under ther the sink. 2) Make sure the circuit breaker at the breaker panel has not tripped. 3) Make sure disposal is off and make sure it is not jammed, 3) Press reset breaker located on bottom of unit (red button.) Then try wall switch again.

you need to be careful you may be getting close to clogging your sink thats really the only time you would get a backwash from a disposal