Never mind. After about two hours of searching, I finally found the answer.
BTW - it's the 2755 HP desktop, not 275. This slow computer takes a while to echo my keystrokes sometimes.
If anyone else has the problem, the "left" side panel has to be removed first, before undoing the thumbscrews and pulling the metal side off. The plastic panel is on the left if you're facing the front of the case, on the right if you're facing the back side. Both side panels come off the same way. First, remove the stabilizer brace from the front bottom of the case. Then pull the center of the side panel away from the case a little bit and push the panel forward. Complete Instructions