Is the USB microphone selected on the Input tab of the Sound pane of System Preferences?
In theory, CrossOver should see all audio input devices similarly. It's possible that the different capabilities of the devices may introduce problems, though.
Can you collect a bit of info for me?
- Launch /Applications/Utilities/Audio MIDI Setup.app
- Make sure that the Default Input pop-up shows your USB microphone
- Select the same device for the Properties For pop-up
Now, let me know what it says in the lower-left part of the window. I'm interested in the values shown for "Source" and "Format" (both the sample rate and the number of channels and their bit size).
If you have options for the Format, you might try varying these, restarting UltraStar, and see if there's any combination which works.