OWC's step-by-step video of how to install memory in the Apple iMac 24" & 20" (Mid 2007 / Early 2008 / Early 2009).
The iMac9,1 model's memory notch aligns in the opposite direction of iMac7,1 and iMac8,1 models.
To ensure your memory installs correctly, please ensure proper memory alignment via the illustration located at the bottom of the iMac's stand. The notch in the memory module ensures proper one-way alignment and installation. After the memory is properly aligned, a simple push is all that is needed to seat the memory into the slot.