Hey Carlisle, I found the ProjecTMix manual online and checked it out, I'm not nearly so optimistic as I was before. I think the external midi port is for midi devices connected to the unit. I didn't realize that it was also an interface (:
I'm sort of struggling to see how it's gonna work in ACT at this point, with the only port it - the projectMix itself - uses being bound to the Mackie plug and disabling ACT. You might could set it up on the control surface port, but as a midi act controller rather than a mackie, and get ACT to work, and have both ACT and Mackie mode controllers at the same time on the same port, maybe, but that would probably cause issues if it worked.
BUT - the manual also said the thing is supposed to be able to control your plugins - in Mackie mode. So I think I should step back to your original posts and ask 1) can you control any plugins with it at present, and 2) exactly what do you want to use ACT for?
Also, even if it controls plugs in the fx bin, it isn't going to control the Pro Channel in Mackie mode, this you may know but its a problem in X1, not specific to the ProjectMix. I worked around it with my novation using ACT because it exposed several midi ports to the DAW, including one dedicated to its control functions like yours. I hoped your external one was something like mine, but alas it's the interface midi.
If you aren't able to control any plugs, even fx bin, then you have a different problem than we've been chasing, I think, as the doc says this should work....