If it's working in others DAWs you should be able to add it via Studio One > Options > External Devices > Add > AKAI > MPKmini. And then wait a minute for it to read the device. I have it set to all channels, Receive from - I just selected the top device (MPK25 since I don't own a mini), left all filters unchecked, send to - none, split channels left unchecked, and default instrument input checked. Click OK. It should show your device listed in the External Devices screen now. It will say "Not Connected" under "Send To", that is normal as your not sending midi to your keyboard. So ignore it. Then on the mixer, click on the speaker of the instrument track you want to play. You might still have to drag a synth onto the instrument track, so you can hear it.
If the external device list is missing in Studio One, I assume you will probably have to uninstall and reinstall Studio One.