If the manual that came with it doesn't explain it better, here is what I found when I googled it:
"View the + sign shaped holes in each expansion slot cover.
These are for you to insert an appropriate flat tipped screwdriver, from the outside. (Or back of computer)
Screwdriver tip inserted, move the screwdriver handle up, and down slowly.
This will break tabs on the expansion slot cover to metal frame.
Just keep twisting the expansion slot cover back, and forth until the tabs break one by one, and remove the slot cover.
(CAUTION! The edges are sharp where the tabs have broken! )
Source Years of computer hardware experience"
link
Hope that helps!
EDIT: Whoops! Ninja'd! ;-)