Setup with USB keyboard and microphone

I am a novice to Mainstage 3 but I would like to use it to some sound experiments, but I would like to be able to set it up so that I can have input from mye USB keyboard (M-Audio Keystudio) and a microphone (either CADu37 USB-mike or Behringer C-1 XLR). It seems like I need to get an audio interface to be able to do that, but it also seems like most only have the normal MIDI In and Out and not USB. If they have USB they are only USB-powered but no USB-connection in.

I also tried to add aggregate device in the Audio MIDI Setup but didn't get it to work. Maybe it isn't for that, either? Perhaps the keyboard should be connected directly to the computer and not through a USB-hub as it is now?

Anyone has a good tip or advice?