FlashDevelop and the fl class package
I'm surprised it took me this long to encounter this problem; but FlashDevelop doesn't seem to know the location of the fl class package by default. I found this out today while trying to use a Color (fl.motion.Color) object. To my surprise, FlashDevelop didn't seem to recognize the import. I tried to build and received an error message that the class couldn't be found.
The fix was pretty simple; I just added the path to the program's global class path list. If you're wondering how to do this, just go to Tools > Global Classpaths, or hit CTRL + F9. Now you just need to enter the location of the fl package, which on my Windows 7 machine is: C:\Program Files\Adobe\Adobe Flash CS5\Common\Configuration\ActionScript 3.0\projects\Flash\src. That's all there is to it; now you can use the classes as you please!