Hi Folks, I hope this post is somewhat related and apologies if it isn't.
I just got an Akai MPK261 and watched the awesome video by Visceral Serenity regarding mapping here:
https://www.youtube.com/watch?v=Kr24vbF4aow
My issue was that I could play the VST instruments but was not able to assign the transport per instructions in the video. After searching, I read somewhere in this forum - and mean to thank who posted it - that just seeing the controller in your MIDI Devices under REAPER Preferences may not be enough.
Once your controller shows up in the MIDI Device, the top section, double-click on the controller's name. Make sure that the 'Enable input for control messages' is also selected. It wasn't in my case (default); I realized that it is vital for mapping the transport button, knobs, etc.
Anyway, it's working now, and I hope this post helps someone else.
Kenny