Filter the BuddyPress Registration page to the Membership Levels page.

my_bp_get_signup_page.php

<?php
/*
  Filter the BuddyPress Registration page to the Membership Levels page.
*/
function my_bp_get_signup_page( $page )
{
	//is PMPro activated?
	if(defined('PMPRO_VERSION'))
	{
		//filter the BuddyPress registration page to PMPro levels page
		$page = pmpro_url('levels');
	}
	return $page;
}
add_filter( 'bp_get_signup_page' , 'my_bp_get_signup_page' );