I have a 2010 iMac formerly running SL. There is only 1 USB port that will properly recognize the phone. Try the USB port right next to the firewire port on the back of the iMac. If that one doesn't work try the others. It won't work thru the keyboard or a USB hub. Also, make sure you use a good USB cable. Mine works perfectly and has worked consistently since I figured out the port.
I've found Bluetooth file transfer to be much more reliable and less hassle than fooling with the USB cable. Too much frustration with the cable approach.