I have always used Lightroom with a PC and now have installed Lightroom 3.6 on my new iMac. I cannot import raw (NEF) files from iPhoto (or from my Lacie drive for that matter into Lightroom. I have tried Lightroom Help to no avail. How do I import NEF files from the iMac into Lightroom?
If they are the same nef files that you were importing into your PC, see if the reason is an error message like the thread above this one. Permissions issue.
What camera are these files from? Lightroom 3.6 is very old and perhaps does not support the camera you are trying to load the files from.