I think all you need to do is click the UserCP at the topleft and then click on paid subscriptions in the left lower corner and go from there. I don't remember having to log back in once on that page. You should already be on your user account and logged in.
edit- I think the comment to add user name (which I just noticed) is for paying by mail in. If you are logged in, you just need to hit the dropdown bar to select $25. Hope that makes sense.