By default, the pads in BPM are assigned to the General MIDI note for the given sounds: BD1 (Bass Drum 1) is C1, SD1 (Snare Drum 1) is D1, and so on. The assigned note is displayed in the lower right corner of each pad.

To assign a pad in BPM to a different note, right-click on the pad and choose "Learn MIDI Key" from the contextual menu, then send the desired note from your MIDI controller.