It is already enabled.  I did some more testing.  I do have the PMDG 737 NGXu, both the 600/700 and 800/900.  I loaded up the 600/700/900 individually, and started MCE.  Worked for all of them.  I then tried the 800 with a different livery (I had been using Qantas livery from PMDG, but tried the Delta livery).  MCE worked.   I then loaded a default plane, and then the Qantas 737-800 again.   Started up MCE.  Got the message of looking for aircraft data again, and P3D CTD.  It seems to be something to do with that Qantas livery.  None of the other aircraft sparked the "looking for aircraft data" message from MCE.  Any ideas?

OK, in the aircraft.cfg file, I looked at the Qantas entry, and noticed that under ATC Airline, it is listed as Shura Transport Services.  Could this be causing the issue?  No idea who that is.

OK, I deleted that PMDG provided Qantas livery, and installed a 738 and 739 livery from Inibuilds.   Both of them now work with MCE.  That livery from PMDG was somehow causing the problem.  Go figure!

9 hours ago, Greg Goodavish said:

It is already enabled.

OK, since you didn't have an issue when "mcfslA3X.dll" (for FsLabs planes support) was loaded, I am guessing you have some sort of issue with Simconnect.

Unlike with PMDG planes where it is used, it's not the case with FsLabs or others like CS planes.