Sometimes, after an installation of an external Magento extension the access to it can be denied in the script's admin area.

As a solution of the “Access denied” issue you should log out from the Magento admin area and then log in again as the script administrator.

If the above does not help, you should reset the admin privileges.

This can be completed through the Magento admin area->System->Permissions->Roles->Administrators.

Click on the Role Resources option from the left menu and make sure that Resource Access is set to All.

Click on the Save Role button and the permissions will be reset.