These are the most frequent causes because a scripting addition does not work:
You are in OS X and trying to use a classic scripting addition (or the reverse situation). Look at Osaxen.com for a list of OS X and classic osaxen.
You didn’t install it properly. Read carefully our faq How do I install a scripting addition?.
You are trying to use classic osaxen from OS X, but you don’t know how. Read our faq How can I use classic osaxen in OS X?.
You are trying to use a embedded scripting addition in your application’s bundle, but it doesn’t work. Read our faq Can I include a scripting addition in my application?.
If none of these solved your issue, most probably you are working with a corrupted, buggie or outdated scripting addition, or with a file which is not a scripting addition at all. Try finding the newest version available and reinstalling it again. If you think the osax contains bugs, remember to report it back to the author.