I need to adjust the memory card in my PC and to do that I must open the computer. In the back a diagram shows that one must first turn the lock to the unlock position (which I did), then slide a green tab to the open position (which I did). My question now is where is the door that opens? Looking at the PC I cannot see a panel or a door that is configured to open. Some guidance, please.

What is the exact make and model of the computer?

There is nothing on the computer I can see that identifies its maker.

Is it a flat desktop? A tower? A laptop?

From your description it sounds like the whole case comes off. If this is the case then you just need to kinda hold the rear panel and sorta tug the case back by grabbing the sides.

Then again some cases the entire side comes off rather than the whole case cover.