Generally on the Futaba radios I used when your looking at the settings in any menu(Trainer included) many are by default Inhibited. When you activate a function the Inh changes to Off or On depending on the position of the switch that is set to work the function. IIRC the Trainer function will not activate if its not seeing a valid signal from the BB.
Which cable are you using? I had the Hangar 9 one I used but IIRC it may not have been bi-directional. IIRC it was designed to use a SPM/JR Master and a Futaba Slave.
Which model of Futaba Tx are you using? I still have a 7C and 9C and the Cable. If I get a chance Ill drag one out and recheck whether or not the cable is bi- directional.
Ive been flying SPM for years now. Its been awhile but I remember having trouble with the trainer system between various Futaba radios. Some combinations would work and some simply refused.