Can it be used as a soft synth (like TTS) where I just tell it to work with a midi track? If so, I don't see any way to accomplish that.
It IS a softsynth.... and a good one.
All softsynths respond to MIDI data. They don't make any sounds without that.
Insert Dimension Pro into the synth rack. If you select Simple Instrument Track from the popup, that's all you need to do. If not, then Dimension Pro will be on a synth track and you need to insert a MIDI track and set the output of the MIDI track to the Dimension Pro.
Unlike the TTS-1, however, DPro will only play one sound at a time so for different sounds, you need to load a separate instance of DPro for each. (That's not totally true, there is an advanced function that will let you load 4 sounds into DPro, 1 into each element. See the Help Menu for how to do that.)
If you really want to get into Dimension Pro, Cakewalk has a video tutorial that is not too expensive and well worth it if you can get past the instructor's Scottish accent.) :)