The S5 Box Login and Register areas are widget positions. WordPress, for some reason, doesn't include a login or register form as a widget. Your best option is to search the
plugin repository for a login/register widget plugin that will do what you want. I have seen a couple that use Ajax to allow a user to log in without needing to refresh the page, but I haven't actually used any of them, so I can't really say how good they might be.