You'll need the MPK25 editor available on Akai's site, or manually switch tracks and use different presets in the MPK. It seems like an empty Drum Rack that you would drag your samples to would be your easiest option to load your hits, and maybe single Simpler devices if you want to load a loop. If you want to slice the loop to pads, right click on the sample (on the Live Clip graphic) and choose 'Slice to New MIDI track', it will map the chops to a Drum Rack automatically.
I have a couple MPD's, but I never considered the issue your having. Most of the presets on MPD's have pad one as MIDI note 36 C1, which is probably the same note as the first white key on your MPK. I'm guessing that's where your problem starts. They probably figured users would either use the keys or the pads separately (like synth on one track and Drum Rack on another). You could use Live's 'MIDI From' selector to disable MIDI input to tracks you're not working with at a given moment, and for example having MPK only control the Drum Rack track. (Make sure In/Out is checked off in Live's View options so you can make these switches)
Alternatively, try going in the editor software and change the pad's MIDI note assignments to higher MIDI notes (ones you wouldn't use on the keyboard so much, like MIDI notes 115-127).
You really don't even have to use the editor, you could press the edit button on the MPK itself, and use the data knob to dial in these higher MIDI values for the 12 pads. Then move the selector box in the right side of the Drum Rack to match the higher notes, and drag your samples to those pads.
I think it's gonna be one these workarounds to get what you want going. Sounds like your just starting out maybe, and MIDI can be a confusing pain. Once you get used to it though it's not too bad, and MIDI mapping opens up all kinds of cool options if you have controller or two.
