If you install a Sametime 8.5 Media Manager you need to specify exactly one deployment name of a Community Server to which the Media Manager should connect to. There is currently no possibility in the GUI to connect to a Community Cluster. You need to do that manually afterwards by editing the following config file:
<Websphere Root\AppServer\profiles\<ProfileName>\config\<Cell Name>/nodes/<Node Name>/servers/STMediaServer/stavconfig.xml
Look for the line
name="STCommunityServerHost" value="<name of you community server">
and change the name to the name of your Community server load balancer.