It's embedded with the plugin. Try rerunning the Indigo installer if you haven't. I think everyone else that's seen the problem has managed to get it to work by reinstalling but it doesn't seem to be predictable in any way. And I have yet to reproduce it on my systems.
I'm quickly running out of ideas. A post on the appscript website doesn't give me a lot of hope that its going to get any better. And I'm not sure there's any realistic way of getting it working without the library. Bottom line is that short of the Airfoil guys building a API into the app, I'm not sure that there's any way to permanently fix the problem. I'll have to spend some time thinking about it some more.