I launch the game from steam, and in the top right corner it says: logged in as RustyMember. When I then pick a character it asks me to log-in, and the log-in button doesn't work (what should i even type into the log-in fields?)
Realm of the mad god is currently having some buggs with steam and kabam is working to fix this.(sure like kabam will do something)
This is the same response I've got from the steam and kabam support.
Also I've found this online:
Have you ever registered an email address to your account? If so, try logging in at http://realmofthemadgod.com and going through the new login/password process. I had to do the same thing and now the Steam version plays as expected.