I think that model of Dell open like a book... The side on the left should have a handle or something on the back, if you open the side, it will hinge towards the front. At least that's what I think.
Check for stickers on the back though, if you open it up and break the sticker your warranty will be void. So make sure you want to definitely open it before you do it.
Yeah, I've seen the top button, but I didn't know there was one on the bottom that had to be pressed with the top one. I've been used to the eMachines format, which has two screw knobs on the back of one side that, after you loosen, releases the entire side panel.