how to turn a too-tight screw
it happened to me, too: i couldn't get the screw to turn, and i thought i'd have to take the whole frakkin' imac to the apple store to install my 3rd-party RAM.
i had used a phillips screwdriver, as instructed. this only softened the cross-hair threading on the screw, and it became even more difficult to turn the screw. it didn't turn a bit--nada.
then i got creative. i took a small flathead screwdriver, angled it so that one corner of the flathead went toward the dead center of the screw's crosshairs. the V-shape of my angled flathead placed the edges of the flathead into two of the screw's four grooves (the groove pair being 180 degress apart). then i pushed really hard to keep the flathead from coming loose and softening the crosshairs even more.
**this is key: push very firmly into the screw while gradually turning it counterclockwise. to keep your imac from sliding away from you, you should stabilize the top end of the mac with your non-dominant hand.**
you won't need to apply as much pressure when you're tightening the screw, after RAM insertion. and don't tighten the screw too much, so that the next time you need to access the RAM door, it won't be such a bitch to unscrew the screw again.
i had a similar problem on my wife's ibook a few years back. i tried to install the RAM myself, even bought special hobbit-sized screwdrivers for the operation, but jacked up the screws' threading. then had to take the ibook+memory in to macmall (i didn't know apple stores would help you for free) and pay $40 to see some dude jam his screwdriver with sasquatchian force into the screw.
a bit long-winded, but i hope this helps.