I'll paste the answer from PL here for future reference:
1. To erase a learned control - first, turn on MIDI Learn Mode - then click on the control that you want to erase - it should turn green showing that it has an assignment. Simply shift-click on it again to erase the assignment.
2. For triggering the presets - you have to use Note On midi messages. There are two modes - Normal Mode (APC Trigger Mode is off), and APC Trigger Mode (APC Trigger Mode button is on).
For Normal Mode, the "Note Offset" number box specifies what MIDI Note # is assigned to the first preset (the preset in the top-left corner). MIDI Notes are then automatically assigned incrementally from left-to-right, top-to-bottom.
This doesn't work well with an APC 40 - as the MIDI Note numbers of the clip-launch buttons on an APC-40 start with MIDI Note # 0 in the bottom-left corner and increment from left-to-right, bottom-to-top. This note mapping is opposite of the "preset" numbers used by Maxwell, which is why there is an APC Trigger Mode - it takes care of this mapping automatically.
3. The waveform control can be controlled by a MIDI CC messages with data in the following range triggering each waveform type as so:
4. At the moment, there is no back-channel communication from Maxwell to a MIDI controller (except with the APC-40 Colorizer App included with Maxwell). If there is enough interest, I would like to add support for a MIDI controller like the APC-40mkii - but that is quite a large development effort.
Note about saved MIDI mappings: These are saved and loaded automatically on app close/open. The mapping files are located in the following location:
OS X: ~/Library/Application Support/Maxwell/
If you ever want to simply remove all MIDI mappings, you can just erase those two files.