It's not always a lever. You may have to undo 2 or more screws to pop off the side panel to your PC. Just look for the side of the computer that looks like it would slide off and remove the screws.
Can you take a picture of the back of your PC?
Geez turn some lights on. Also, see that knobby little bolt on the left side? Is that unscrewable? Also check to see which side of the computer looks like it can slide off.
I had a light on and I don't think it's unscrewable, just tried :cunless I need a screwdriver.
If it looks like a phillips head or flat head you probably need a screwdriver. :/