Freenom (aka dot.tk) - Changing nameservers?

Does anyone here have a freenom domain (.tk, .cf, .ml and so on)? I seem to be unable to change the nameservers an an already-registered domain, it keeps directing me to the login/default page...
I assume it's a server-side bug, but it could just as well have to do with my browser, I'm wondering if anyone else has had similar issues and whether they've managed to get them changed...