In Studio One, the track fader buttons display "Channel Unknown" when I try to use them.

You must have a track selected in order for it to activate. 

You can use the track selector and in coordination with the bank buttons to navigate to the right track, Note that the MCU or Mackie protocol works on 8 track selection per bank. So if the selected track is outside the selected 8 tracks bank it will be an unknown channel. The plugin does not recognize that. But if you are on the right bank and yet no track is selected in that bank still it wont recognize. 

You will see the track names in the lower portion of the Fader, or the Arm, Solo and Mute

If you see the track names that means you have a selected track controllable by the fader, Pan, Solo, Mute and record arm.