Take it in
You shouldn't access the slot yourself... Doing so detaches crucial thermal sinks in your case, which many here have let me know when I asked the same question. You will need torx screwdrivers, the ability to live with voiding your warrantee, and special Thermal Pads (which MUST be replaced if you open your imac beyond the user serviceable panel!!!). Once you have all of that then there is a slot on the motherboard for a standard PC100 SDRAM chip (up to 512mb).