Hi, dom!
To adjust the volume of all Multipads (at the same time, they can't be adjusted individually), you have to go to the "Panel" page of the Mixer, not to the M.Pad page (as you show on your image). The second entry in the Panel page is "M.Pad", which allows you to control all the MultiPad channels at the same time. Here is the only place in the mixer where you can adjust this...
The settings you can modify are: volume, equalizer low, equalizer high, Pan, Reverb and Chorus, but for all the Multipads simultaneously.
Hope this helps. Regards!