OPENING UP AN IMAC IS DIFFICULT
From the very first iMac, the level of difficulty to open the system started off difficult and become more complex. With each succeeding generation, the process became more and more difficult.
Newer Macs use adhesive strips to hold their monitors in place, so you need the proper tools in order to open it up.
REMOVING THE MOTHERBOARD
Once inside, you have to remove most of the iMac's guts in order to remove the motherboard. This task requires other specialized tools and you need to keep track of the different size screws which Apple used.
Once you have the motherboard out, the memory sockets are located on the back.
PUTTING IT BACK TOGETHER
Once you have the memory installed, you then have to put the whole system back together, first with the motherboard and then placing each part back into the Mac in the opposite order.
This proceedure also requires you to have new adhesive strips so that you can reattach the screen.