My client has enabled webalizer, but when he tries to access it through http://domain.com/webalizer/ he gets the error: “Forbidden, You don’t have permission to access /webalizer/ on this server.”

This can happen if you set a directory index in the root folder to something other than html, shtml etc. You can either put the index.html on the end of the url, or put a htaccess file in the webalizer folder setting index.html as the directory index.