the default sign up button now points to:
I need to change the url so that it points to the real sign up page.
I looked at register.php, but I don’t know php well enough to figure out how to do it. I searched for the url with firebug, but am still unclear on where exactly I can find – and change – the url that the button points to.
you need to look into the sourcecode and find the place where that link is created. I do not know WPMU/Buddypress specifically enough that I can answer this question out of my head, but basically you’ll have the following options:
Firebug can help a bit to analyze what’s going on, but it’s only in the browser. Changes need to be made on the server side so you should have at least some PHP and WordPress experience.
Or to find a plugin that does the job and you’re lucky. But I’m not aware of such.
BuddyPress employs a different signup/registration process (creation steps, xprofile fields, avatar, account activation email, etc)
You could either create a child theme for registration/register.php and make the appropriated edits for your needs. Hook into the actions to add more content or create additional steps.
Or copy some of the source code which is required by BuddyPress