A-Tech 4GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010, Mid 2011, Late 2011), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com
How to upgrade RAM on iMac mid 2011 (from 4GB to 12GB) - YouTube
How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube