what have you got plugged in to the UX1..guitar or mic ? As for either you have to identify it in POD Farm in the drop down box, by selecting 'instrument' if using guitar or 'mic 1' if you are plugged in to the no.1 input on the UX1 etc...
Also you might need to change your System Preferences on your Imac, in the Devices you should see the Line 6 device, you can alter input and output settings here which might help solve your issue.
Sorry cant be more helpful, I am at work on a PC and not at home with my Mac to be more detailed and specific.
There are more folks on here who can probably help a lot better but thought I would get the ball rolling !
Best of luck