The Core Duo iMacs have two RAM slots under that door on the bottom. As Masna said, remove two screws, remove the door, take out the stock RAM, and add the new RAM. Piece of cake.
Be sure to hold onto that stock RAM. Apple has a history of blaming user installed RAM for problems should you have to take it in for service. Besides, if something does happen to your Mac and it needs replacing, you don't want your RAM dissappearing.