[spkguitar]

Without more specific details, (version of Pro Tools? version of DP? Version of OSX?, etc) it's hard to be specific.

What I'm assuming you are doing is selecting "Instrument Track -> Apple -> AUMIDISynth (stereo)" in DP. In DP, "Instrument Tracks" use plugins to produce sound, either "AU" or "Audio Unit" plugins (of which the Apple MIDI synth is one) or "VST" plugins depending on how you have it configured (and possibly which version you are using).

Pro Tools uses it's own proprietary plugins (either RTAS or AAX depending on your version or PT) and cannot use the "AU" or "VST" plugins directly without some kind of "wrapper" (converter), or by using another program (like DP) in rewire mode.

So in your case, what is happening is that while in DP, you can directly access the Apple synth as MIDI output because DP can use AU plugins, but because Pro Tools cannot access the AU plugin directly, you can't access it directly, and would have to either use it as you mentioned (via rewire), or use a third party wrapper (converter) for whichever other format plugins you wanted to use. Recommendations could be made for you, but we would need to know which version of PT to know which to suggest.

I hope you find this helpful.