AirFMC says default B738 in XP11 is "incompatible aircraft".

Unfortunately yes, for now :(

It is compatible with the x737 from EADT and with the 737 from IXEG but not with the new 737-NG in XPlane-11

Reason being, the 737-NG (as well as other new planes in XP11) don't currently have a way of showing of what's on the MCDUs (i.e. doesn't have these datarefs) so that AirFMC (or the plugin) can read its lines and show them.

I'm currently in contact with Laminar and they have this planned in their roadmap, so as soon as this happens this plane (and possibly others) will be supported by AirFMC and I'll adapt the plugin.

Should happen early 2018, sorry for the inconvenience in the meantime,
joao @ haversine

Okay, thank you.