Where is the heater case door located on a 92 Nissan Sentra?

could be but before you jump to conclusions, get an opinion from a shop. it may be a clogged heater core and flushing it may be the answer.
dont do needless work unless you are sure.
it is under the dash on the passenger side and is not that easy for someone at home to attempt without proof.

Roy

Should I flush the radiator while I'm at it? Just trying to pin point why I have no hot air when I have the heat on...it's going to be getting cold and I don't have money to spend on pricy repairs.

yes, always a good thing to flush the complete system

Roy

If I do this and still no heat what would be my next thing to check?

lets take one step at a time. since you are doing this yourself without a shop doing it, the flush you will do may not unclog the core. it has to be power flushed, that is a machine the uses pressure to remove the debris that has the flow restricted.

try it first before we go any further.

Roy