Shoutbox

Fring, like many other apps, don't support Windows Live Messenger's Multiple Points of Presence (MPOP) feature, which does not have any relation with Messenger Plus! Live.
What would solve the problem is contacting the developer, or their official comunity (which happens to be on http://www.fring.com/forums/ ) forums, ask them to implement this feature, and wait until, and if, it happens.

Sorry, but Plus! can't help you with this.