Howdy @bsohnlein - My guess is that your iMac is not recognizing the D3100 when plugged directly into the USB port. You'll need to install the ViewNX 2 software that came with your D3100 or you can purchase a memory card reader, plug it into an available USB port and then insert the memory card. Happy shooting! :)
iMac has built in card reader. it is on the right side just below the cd/dvd slot. Plug the memory card in there and then open Finder. You should see your memory card listed there. If memory card is new and you did not give it a name then you shall see it listed as "NO NAME".
I noticed my D3100 doesn't show up in the Finder either and my newer card reader doesn't support my brand new card. The camera does show up in the import screen of my Lightroom 4. -c-
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-c-