How to integrate upgrade link/form for a Pro Sites from a free one?

Ok. I have Pro Sites installed, have tweaked the wp-signup.php page to suit my verbiage and liking (for the most part), and have the site creation set up.

I would ideally love a setup that allows a customer to create a user, site, select Pro Site package, and pay (all at once) - which would then grant access to their site. Since that appears to be a massive headache from what I've found on these forums, this is my second option.

The "have-to-because-of-above" option:
- user to create free site, which will always be the same...a "welcome" one-pager, with tutorial videos on how to use my service (which requires a Pro Site), and helpful pre-upgrade support questions. This should be the same for everyone who sets up a site.
- They will have an "upgrade" button on the one-pager and the nav for that page, shown at all times for quick access.

My question is this:
What page actually pulls up the PS upgrade options/form? I don't see it on the settings area, and the upgrade button is not appearing on my top bar like I've heard it should (settings are correct for that).

Looking forward to the Pro Sites upgrade, and hopefully it will come soon. The potential is great for it, but it's an awkward setup how it is. I am not alone in my thinking this (from the forum posts). Regardless, it is a very cool plugin that will come in handy.