Exhange Instant Messenger

We have three locations. Each on a different domain using W2K Active Directory and a different subnet. The locations are connected by frac-t1. I have recently installed a new Exchange 2000 server at the primary location. I would like to use the instant messenger server that comes with it. However, I cannot get the other two locations to see the exchange server and hookup to the IM server. Does anyone know what I need to do to get all three locations to use the same IM server?