I had posted another reply, but suddenly the site said "we'll be right back"......
In any case, I just tried to check the specs for the 2010 iMacs to get some insight into the SD card slot, but couldn't find anything. I was wondering if your card is compatible with the Mac slot.
So, if yours won't read the card, you can either get USB reader, use a firewire 800 > 400 cable, or - and I would recomment that - take advantage of your warranty and either call Apple or make an appointment at your nearest Genius Bar. I am assuming you are still covered by the warranty if you have a 2010.