Akai MPK 261 …..Cubase showing MIDI but instrument not playing

So I have my Akai MK261 keyboard plugged into the midi input of my RME Fireface 400 MIDI input No: 1. I can see the MIDI output in the transport bar of Cubase but it is not triggering the instrument? Any ideas chaps?

The MPK261 has MIDI over USB. Have you tried that? I don’t understand why you want to go via your Fireface.
Can you give a bit more info about your setup?

OK, Mac MINI OS X Cubase 11 RME Fireface 400.
I have the MK261 hooked up with USB (powers it too) but still no triggering of software instruments. I can see the MIDI thru registering in the transport so its getting signal.
I hooked up the MIDI out of the MK261 to the the RME Fireface 400 to see if that would get it…but no.

Difficult to tell what the problem is. First of all I recommend NOT to connect the MK261 to the Fireface with a DIN MIDI cable if the MK261 is connected to the Mac via USB at the same time. (This kind of thing got me in trouble as well)
Then check these things in Studio - Setup: Audio system : Is there a driver selected? I guess the Fireface comes with a driver, if not, choose something with ASIO if available.
Check in Studio - Audio connections: do you have inputs and outputs assigned?
Then: Create a new Instrument track with your VST of choice. By default, this will be set to ‘All MIDI inputs’ which is OK. Make sure the VST is set to an instrument/sound or whatever that produces noise for testing your setup.

I tried all of that to no avail. What isn really annoying is that it works perfectly in Logic Pro X!

So you have not set it up correctly in cubase.
Could you describe your midi routing completely, please?

I have the MK261 connected via USB (so it powers the MK261)

Understood - and you midi routing in cubase is…?
Please give an exact description of your midi routing, tracks, vst, etc.