If you're testing in adl right now, make sure you've extracted/unpackaged that fullscreen ANE into your "extensions-extracted" folder. So for example, in your extensions-extracted folder you should have a folder named "fullscreen_extracted.ane" (it can be named whatever, as long as it ends with ".ane" in the folder name), and inside of that folder you should see the META-INF directory, catalog.xml, library.swf, and mimetype. If you're seeing something different, that's why that error would be happening.
When packaging with adt, your "extensions" folder should just have all of the original packaged ane files in it (AndroidFullScreen.ane, etc.)