Microsoft Messenger for Mac 7.0.2 and Microsoft Communicator for Mac 2011 are two instant messaging clients that are designed for use with the Apple operating system in an Office Communications Server or Lync Server 2010 unified communications environment. This combination of technologies requires user knowledge of the preliminary steps that are needed to get Mac IM clients connected to the Windows unified communications environment. This link to the article troubleshooting steps that you can use to test Mac connectivity to the Communications Server or Lync Server 2010 pool servers.

http://blogs.technet.com/b/nexthop/archive/2010/12/13/troubleshooting-connectivity-for-messenger-for-mac-7-0-2-and-communicator-for-mac-2011-with-communications-server-2007-r2-and-lync-server-2010.aspx