In order to logon to Office Communicator 2007 you must first deploy Office Communications Server (OCS) 2007 or have a Live Communications Server 2005 deployment in place. Office Communicator 2007 does not have the ability to logon to public instant messaging services like Live Messenger as it is for use internally by organizations (and between organizations) with their own managed IT infrastructure.
Here's some information on OCS to get you started:
http://office.microsoft.com/en-us/communicationsserver/default.aspx