I'm afraid all I can do is confirm what the others have told you. That mixer only outputs two channels (i.e. the L/R main mix) to the USB connection to the computer.
I will, however, add a comment that I wish manufacturers would stop being misleading (or at least vague) about the routing capabilities of their mixers. There's no technical reason why individual channels can't be sent to a USB2 or Firewire interface--there a a small number of mixers that do exactly what you want. However, they cost rather more than entry level mixers like your Behringer. If you happen to read this, Uli, please make it clear on your features or specs list that your mixers do main mix only to the USB.
Also they often create more problems by routing the computer output back into the main mix which is another problem...but one for another thread.