Thank you for your post.
The 3rd party plug-in would need to be in the Extensions folder. On our web site, you can view our product documentation for FileMaker Pro 9 Advanced. Look at page 13, right column for more information.
You only have to purchase one copy of FileMaker Pro 9 Advanced. The other users do NOT need FileMaker Pro. When you distribute your solution, you would also need to distribute the 3rd party plug-in.
Please see the documentation above for more information. If you still have outstanding questions, please feel free to contact me.