I just bought my Tritton AX720 a couple of days ago. Like you I tried to use the USB cable to connect it to my MB Air in order to communicate via Skype, but I cannot get it to work at all.
Changing the settings to the C-Media USB Device in the Skype- or System settings doesn't work, all the sound is still played and recorded with the built-in speakers/mic.
I changed the settings in the Audio-MIDI setup-menue, which resulted in the sound being played via the Headset, but the mic didn't work.
Furthermore: changing the Audio-MIDI options back to the built-in hardware doesn't work - the sound is still played via the Headset and the built-in mic cannot be activated. Only when i unplug the headset can i use the mic again.