Hi! You should create pages in dashboard and name theme : Activity, Groups, Forums, Members, Register,.
Be sure You have PERMALIKS ON , theme folder include BuddypressTemplate Pack files and on editing page below title is right url (example: [ Permalink: http://natfriends.com/site/members/%5D )
Now i trying this at localhost. Is working (WordPress 3.1alpha BuddyPress 1.3-bleeding )