Hi,
Congratulations on getting the new Aria player - I have it also, and find it to be much easier to work with than the older Kontakt versions of the Garritan player.
However, as you've noted, it is no longer it's own "MIDI device." Fortunately the solution is pretty easy. You just need a set of "virtual" MIDI cables. Probably one of the easiest and most versatile (and free!) is LoopBe, available at
http://www.nerds.de/en/loopbe1.html Unless you use more than 16 instruments at a time, that's all you'll need. (There are other free virtual MIDI cables, and the LoopBe ones are the most trouble-free that I've come across, but the free version is a single "cable". If you need more and want them free, you may want to explore other options such as
those listed here.)
Install the LoopBe "cable", then run Notation Composer. You'll see it show up in your list of MIDI Devices. Run Aria, select "Tools", then "Preferences", and you'll see it as an option there - just check the box.
Enjoy
ttfn,
Sherry