You may not be able to use the USB headset to "monitor" what's going to your stream -- but make no mistake -- the record mixer is conrolling what is going there.
If I were you, I would het a set of real headphones, and plub them into the speaker out on your soundcard, and plug a real MIC into the MIC input of your soundcard, and try again.
USB is a "hack" -- many times it does not work the way real traditional devices work, this is true for audio devices, and expecially when you try to use USB as a network device -- Your SoundCard only knows what's plugged into IT, Some USB audio devices only use USB for power [most speakers for instance], if your USB is sending audio though the USB interface, you may have to take extra steps to make it work with ShoutCAST -- I would check the documentation for the device, to see if it created yet another mixer device that you may need to select before you can control its output.
I would also look at both mixers [options, preferences...] again -- you may be missing something -- at least once, "check" every input, so that you see them all -- the USB MIC may be there [under a rediculous name].
Your headphones may also part of be the problem -- At least once, try using standard gear before trying the USB -- this will get you familiar with the operation of the mixers.
ps: The link required a login and passord from me.