I was having the same issue running Premiere Pro CS6 picking up my Audio-technica AT2020 USB microphone…
After some trial and error I found the issue was that I had not set an output in my aggregate device in Audio Midi setup. Follow the directions to setup new microphone in Audio MIDI from Adobe’s help menu (As follows):
Set up a USB microphone (Mac OS)
In the Audio Midi Setup application in Utilities, go to Audio > Open Aggregate Device Editor.
Check each of the following:
Your USB microphone.
An additional input device, for example, Line In.
An output device
Select this new aggregate device within Premiere Pro’s Audio Hardware Preference panel.
Once you add the input AND the output in the aggregate device manager you will have to restart premiere and then your microphone should show up in the default devices drop down under the name that you gave the aggregate device in the Aggregate device editor.