BuddyPress.org

@balinic

Please answer DJPaul’s questions as well.

You must have proper rewrite rules in a .htaccess file and mod_rewrite enabled. If you successfully installed WPMU, then it automatically creates a basic, working .htaccess file. Check to make sure that that file exists. Since it is a hidden file, you will need to set your file browser to view hidden files. If it does exist, then next thing to verify is that your hosting provider has mod_write enabled for the Apache.