Does your version of Cubase come with a manual? I would suggest looking up selecting interface devices.
I'm not hugely familiar with Cubase. I haven't used it regularly since 2005. I do also have a degree in Music Technology but that doesn't mean that I know all of the answers, I'm afraid!
This might help:
Replace USB input with your MIDI cable and replace the keyboard with the drum kit. Off the top of my head, percussion is usually assigned to MIDI channel 10 - but I haven't used MIDI inputs for percussion in a long, long