FSX doesn't support a "real" TACAN, meaning: a navigational aid that shows distance+direction on military airplanes, but just distance on civilian equipment.
FSX makes all TACANs to appearing as DME, the F/A-18 TACAN is still using the FSX standard radio, the only added programming is the interface to select by channel, but is still working as a civilian radio, so it's just DME.
This is a Flight Sim design limitation of the navaid system that it has been like this since version 1.0...