In order to assign the drum pads to control Tonic or assign any other function, you will want to change the drum pads so that they are sending MIDI CC Data (preferably on a different channel than what the keyboard keys are sending on too).
You will also want to make sure the CC # you are having them send is not a common CC # for other uses (I would use #'s ranging from 14 to 31). If you need more help with this, check in your user manual and that should help.
After you have done this, you will want to assign the drum pads in the MainStage Layout mode by selecting the button for Tonic you'd like to assign for each.