I finally figured out why help is skimpy on Wix (the Windows Installer XML toolkit): that’s mainly because the original designers of Wix don’t want you to use Wix. It’s that simple: Wix’s learning curve is so steep, I wouldn’t even recommend it if you’re not willing to spend time at it. Additionally, don’t bother looking for sample templates (that is if you can find them) or help for the cryptic warning and error messages.
On the good side, there are a couple of good tutorials that should give you some leeway in your custom install script: There’s Gábor DEÁK JAHN’s (excellent) basic tutorial on Wix, which is probably the first site you should take a look at. Additionally, for more advanced application and use of Wix, you should probably check out Alex Shevchuk’s pages: his articles are highly technical and you may actually pick up some stuff regarding the inner workings of Microsoft’s Setup Installer SDK.
Anyway: if you just figured out that you need to add an additional data directory to your application setup program and you ran into a roadblock of cryptic messages (don’t bother going in here if you’re not interested):