Since it is urgent, I thought I'd wait until later in the morning to respond.
I have found that, for reasons I've never figured out, I can have aMSN work for MSN video chatting with one friend, and not Mercury Messenger, and for another friend, without changing any setting on my computer or network, Mercury works and aMSN does not. Interestingly, usually one or the other will work for me without opening any firewall ports.
The only explanation I can think of is that they do not handle MSN's fallback mechanisms (for if the firewall settings is interfering) in the same way.
So you might try Mercury....