Hi, I suppose you want to playback sequences on your X3, with the Mac controlling it. You should switch to SEQ mode (but in COMBI mode it also works) and supposedly, set your CLOCK settings in GLOBAL mode to EXT (that is what I was told to, but it also works in INT mode).

I do not know what program you are using, but with my PC I usually have to mess a while with the MIDI settings, Setup or however it is called, in order to get the MIDI output right.

Cheers