First let me say that I'm a very new Mac user. I recently bought a new 15" iMac for home with the standard 256MB Ram and I ordered an additional 256 from Crucial. Installed the RAM before hooking up the iMac, and thought everything was well. Except today I actually looked in About This Mac and I saw that it's only detecting 256MB of RAM and nothing in the second slot. I followed the instructions correctly when I installed it, everything snapped into place correctly. Am I missing anything?