Support for the scripts you install on your account would be done through the script vendors homepage.
I am however a user of phpbb myself, so I can give a few pointers.
1. Make sure the style is actually compatible with the version of phpbb you have installed
2. Make sure the style is uploaded correctly, ie: phpbbROOT/styles/stylename (make sure there's no double folders like /styles/stylename/stylename, which is not unheard of when you use the server to unzip the style).
3. If both are correct, you should be able to see the style under the customise tab in the ACP. If you are sure both above is correct and still don't see it, you would need to seek support where you got the style.