I've done this twice on my 27" iMac before. Unplug it and hold it on its side with the disc slot facing down. I rested the edge on a table so the disc slot was hanging over the side. Then insert a thin piece of card, wiggle it about a bit until you can see the SD card then you should be able to get it out with tweezers. Stupid place to put it right next to the disc slot...