If you use the Layer Selector module, you can assign the layers to MIDI notes. The layer will stay switched on for as long as the MIDI note is held down, and switch off when the note is released.
However, this "layer-plays-only-while-note-is-held-down" might not be the behaviour you want. You might prefer "MIDI-note-toggles-layer-on-and-off".
To do this, create a one bar MIDI clip in Live that contains the MIDI note you have assigned to the Layer Selector. Make the MIDI note one bar in length. In the pic below it's MIDI note 1 (C#-2), you can see it in the piano-roll editor. Switch ON loop for this MIDI clip, in the Notes Pane of Live's Clip View.
In the Launch Pane of Live's Clip View, set the launch parameters to
Launch Mode=Toggle and
Quantization=None.
When you trigger the clip inside Live it will turn the layer on, and when you trigger it again, it will turn the layer off.
In the pic below I have assigned this "triggering clip" to receive another MIDI note from my Xio keyboard (channel 1/C3), so that I can have this "toggle" behaviour live under my fingertips.
Hope this all makes sense! I'm new to M8 but old with Live, so there might be a simpler way to do this in M8!