When I upgraded to Studio One Version 4, the transport controls on the M-Audio Axiom 25 I'd been using flawlessly since Version 1 quit working. I finally figured out the fix, I haven't seen collected all in one place.

The bottom line is that while there is an entry already in the "Add Device" dialog, it doesn't map the transport buttons correctly. So you have to add it as a "Generic MIDI Keyboard." Then select it from your external devices (the tiny down arrow just to the left of where you select the Arrow Tool) and click "MIDI Learn mode" near the top of the window. (This option isn't available if you set it up from the factory-supplied Axiom entry).

Click each button on the Axiom's transport controls, and a new MIDI controller entry will show up for each one above the on-screen piano keyboard inside the "External Devices" window. Right-click to tell Studio One that it's a "Button (on/off)," then right-click again and choose "Assign Command" to tell it what you want the button to do. There's a group with all the choices you need way down the list under "Transport," but I think it's easier to search for the command you want.

That's it. I tried to upload the .device file here, but that's apparently not allowed on the board. Anyhow, I hope this helps someone else, because it frustrated me a lot.