I have a 96 Nissan Sentra 1.8 liter I was going to adjust the timing with a timing light where I marked on the harmonic balancer it would not come into view no matter which way I turn the distributor why is this??

Bring #1 cylinder up on compression stroke, TDC this will tell you which mark to use.

most crankshaft pulleys ( harmonic balancer ) are keyed onto the shaft and held there with a bolt
in that case the timing mark will always be in the right place
otherwise set the no 1 piston on tdc (remove no 1 spark plug and with a bit of light gauge wire move the crank until the piston comes up and then starts to go back down)
the mark on the balancer should be at the tdc mark on the timing case cover

On older vehicles... loosen the bolt holding the distributer and turn a little clockwise or counter clockwise. If you have a timing light plug it into the spark plug wire of the #1 cylinder and look for the timing marks on the front of the harmonic balancer. But you could just wing it without a light.

For adjusting the ignition timing, you need a timing light.
Attach the spark lead from the timing light to the number one spark plug.
If the timing light is a powered type then
Attach the power leads of the timing light to the battery - red lead to the + terminal on the battery and the black lead to the - terminal on the battery.

Loosen the distributor so you can adjust the timing, start the engine and press the timing light trigger and aim the timing light on the timing marks on the crankshaft pulley. Adjust the distributor to the recommended timing mark for your engine.
Turn off the engine, remove the timing gun and tighten the lock on the distributor.

The harmonic balancer is integrated with the crankshaft pulley, and if replaced requires taking the bolt out from the side of the 4 cyl. engine.

I would recommend that an experience engine rebuilder perform the service.

Inspect the Harmonic Balancer:

The harmonic balancer (vibration damper) is inspected for signs of wear in its center bore.

• Also inspect the rubber mounting for indications of twisting and deterioration.
• The outer ring can slip, causing an out of balance condition. Replace any harmonic balancer that has slipped.
• With the cylinder head removed and the piston at top dead center (TDC), make a visual comparison. Do the timing marks for TDC on the timing cover and damper appear to be accurate.
• Be sure the damper is the correct one for the vehicle. The wrong one can be worse than none at all.
• Sometimes a damper can become loose on the crank. Check the keyway for wear that can result from improper damper installation, the wrong size key, or a loose damper hub to crank fit.
• Be sure all pulleys are straight. A damaged pulley can force a crankshaft to bend during engine operation.

This is a general guide on replacing the harmonic balancer, but there are other instructions that the rebuilder would follow to actually do the service for you:

To remove the harmonic balancer, the cylinder head and timing cover must be removed and the piston should be at top dead center (TDC).

• Install the vibration damper by pounding it on, or use a special installation tool.
• Generally, the damper is installed unit it bottoms out against the oil slinger and the timing sprocket.
• If in doubt, install the water pump and pulley, and then pound the damper into place until its pulley aligns with the water pump pulley.
• It is advisable to use the installation tool, because pounding on the damper can cause stress on the crank thrust bearing surface.
• It is best to stand the engine block on end and support the crankshaft if the damper must be pounded on.
• Some vibration dampers are not pressed-fit on the crankshaft.
• Be sure to install a large washer behind the damper retaining bolt on these engines; otherwise, the damper might fly off, causing damage and a safety hazard.

timing cover around harmonic balancer just my 2 cents

a drain and fill is about 3.5 litres.if you pump it out completely including the torque converter,about 10 litres

Step1 First step is to open your hood on your vehicle and locate the distributor. Step2 Next loosen your distributor just enough that it will turn from side to side with some resistance. Step3After Loosening the distributor take a piece of chalk and mark on theharmonic balancer the timing mark so it can be seen easily. Step4Then take the timing light and connect it to the battery and the Numberone spark plug wire. Then have your helper start the vehicle while youshine the timing light on the harmonic balance and the timing mark onthe timing chain cover. Step5The Timing Light will illuminate the mark on the harmonic balancer whenthe Number one cylinder is firing. The mark should correspond to thetiming mark on the timing chain cover. You can then adjust the timingforward or backwards by rotating the distributor ever so slightly. Step6 When you have the timing where you want it then shut off the engine and carefully tighten back down the distributor without moving it.

the timing chain tensioner is hydraulic and either out of adjustment or defective.  You need to remove the timing chain cover and reset the mechanical correlation between the crankshaft and the camshafts.  also the other ignition time marks are on the crankshaft harmonic balancer and the cam cover.  Do not crank the engine with the timing out of sequence or you'll destroy the engine.