I would suggest that for Mac Mini, you better send to Apple Centre in case you wanna install additional RAM. I tried to open mine, but it d**n hard, I guess you got to have special device to open it, Do check it here : http://www.ifixit.com/Guide/82.13.1.html.

But for Imac, its pretty easy to install same goes for PowerBook. (Note : for Powerbook user, please take out your battery first before you install any RAM, or else you PowerBook will act strangely after that- coz I have experienced myself)