Two things you definitely want to check before trying to drive around or adding more fluid. First, make sure the linkage is actually moving the shift lever on the transmission. With the button pushed in so the detents don't prevent you from moving the shifter, you should still feel 'bumps' as the shift lever inside the car moves the shift select lever on the transmission. If they're disconnected, you can move the lever inside the car all you want and the transmission won't notice.
Secondly, with the engine idling, check the dipstick on the transmission. It's on the passenger side of the car, up by the firewall. The dipstick should read between the clear and simple-to-see marks. If it's too full, your fluid will get foamy and cause trouble as it is aerated by all the moving parts. It will probably leak, and since foamy fluid compresses (unlike normal transmission fluid) your car will not shift properly. If it's too low, your car probably won't want to go into gear at all. If there's no fluid on the dipstick, add one quart at a time till you see it on the stick. Then add half a quart at a time till it reaches the proper marks. Your fluid should be bright sparkly red. If it's not transparent, has a rusty or brown color, and/or smells burnt, your transmission may need to be serviced. It certainly needs the fluid changed. It is possible, and even likely, that if it's malfunctioning and has nasty looking fluid, changing it will not help your transmission out. That's not the fault of the new fluid though, just an indicator that your friction materials need to be replaced, or that something's got a problem. It is a good idea to always use Type F fluid in C4 transmissions. Modern friction materials do not all require it, but it has some advantages over a lot of the new fluid types, like anti-foaming agents and a friction modifier that will help give your car strong, positive shifts. (GM guys have known for years that putting Type F in their TH350 and TH 400 transmissions is a bit like putting in a shift kit.)
If you have original friction materials, they require Type F, and using any other kind will probably make it slip until all the friction materials fail, even if you later add more Type F.
Hope this helps!