Whenever you install a new module in the admin side of your store you will need to enable this file for use in the Admin Levels which is in osCMax. This always seem to cause a great deal of confusion and is a regular question on the forums.
To add access to a new php file in the administrator menu you need to enable file access
Open your admin panel
Click on the Administrator Menu box
Click File Access in Menu box
Select the relevant Menu Box row in the central box ie. the one you want to add the file to
Click Store Files in right hand column
Select <new_php_file>.php from the drop down
Click Save