modifying the signup button on bp-corporate theme

Hi, I'm using the bp-corporate theme and my site offers paid signups only, but I'm using a different payment system besides the Supporter plugin/PayPal and have the built-in WPMU signup process disabled.

I want to have the signup buttons link to a different URL rather than the normal signup page. I found where to change the link for the big "Join Us Here" button (in the call-signup.php file), but there is also a Sign Up button on the bar that contains the member search box and the login box. I haven't found how to change where that Sign Up button links.

Could you please point me in the right direction?