The thin iMacs didn’t have any room for that, so Apple added a memory access door to the back of the machine, but only on the 27-inch models, as noted by this support article: Memory is not removable ...

This knowledge base article contains all the specifications of this new iMac but interestingly this model (which is dubbed a ‘Late 2011’ iMac) does not have Thunderbolt, that makes this iMac the first ...