It often, that Access denied appears in the Magento backend, after installing an extension.
In this case, just above the right from the backend log out and login again. Should the problem be solved.
Otherwise,: Cache learn bzw. during installation and change to the side at least partially if not completely clear.
Background simplified: Magento works with permissions i.e.. many functions and menus are enabled permissions for users on the backend. Has a newly installed extension on such grant opportunities, these are the only users granted, if he is- and logs in again.
