Usage reporting is a service that enables site administrators, site collection administrators, and Shared Services Provider (SSP) administrators to monitor statistics about the use of their sites. Usage reporting also includes usage reporting for search queries that can be viewed by SSP administrators for search and site collection administrators.
To configure usage reporting, a farm administrator must first enable Windows SharePoint Services usage logging for the farm that hosts the Web application containing the SSP. The SSP administrator enables and configures the usage reporting service. Then, site collection administrators can activate the reporting feature to enable usage reports on the site collection.
I got the forbidden error when I tried to enable the "Enable advanced usage analysis processing" under SSP, eventhough I am a sitecollection /farm admin.
To fix it.
go to SharedServices --->"Personalization services permissions" under "User Profiles and My Sites"
Add yourself with "Manage usage analytics" permission.
Done! Now you should be able to enable it.
If you want to know how to Configure usage reporting go to:
Configure usage reporting