Just FTP into members.gamedev.net, using your regular username and password, to access your personal space. Old GDNet+ members should find all their files ready and waiting for them. We've also increased the space quota to 100MB per user, and we'll look at increasing this further as things get settled in.
Anything you upload into your webspace is accessible over HTTP, too...
Old GDNet+ members can browse to the same addresses that they've always used. We'll probably be retiring these addresses at some point, but we'll make sure we let you know before we do.
New GDNet+ members, log in, and browse your way to http://www.gamedev.net/subscribe - take a look at the bottom of the page for your address info.