Thanks Nealius, but I already had selected A/A T/R-mode selected.
I did manage some progress : I went into the "advanced waypoints" of the tanker in the ME and fiddled with the radio frequency and now it seems to work. Has TACAN anything to do with the radiofreqiency (150 AM or FM ) ? I also saw that in the vriefinf it says 150FM, while in the ME it says AM and in the advanced waypoint settings" it was FM (or AM can't remember). Might have something to do with the 150MHz-radio-threat someone else just started.
Anyway, TACAN worked this time, but I couldn't talk to the tanker.