If you haven’t already, copy bp-themes/bp-default and /bp-sn-parent to the normal WordPress theme directory (/wp-content/themes).
/wp-content/bp-themes/ is for a previous version of BP themes. Unless you know why you might still want to use it, make sure that /wp-content/bp-themes/ does not exist (i.e. delete the folder) else BP will get confused.
Activate the bp-default theme, put it on your main WPMU blog and it should work.
EDIT: Jeff won!