Jmdns is a multicast DNS implementation for Java and is used for service registration and discovery. It is available under Apache license and is included within ATG for the same purpose. (DAS\lib\jmdns-20.jar)
There is one specific requirement for Jmdns when running ATG on WAS under AIX as mentioned here:
But since you are on JBoss so I don't think it should be applicable in your case. One possible cause of /atg/dynamo/service/jmdns/JmdnsService failing to start could be due to some exception occurring during the socket creation within JmDNS implementation class. There can be some network/port issue. Try enabling loggingDebug for /atg/dynamo/service/jmdns/JmdnsService and see if you can get more information or exception stack-trace on this.