I ran into the same issue after installing 3.0.0 - I was so used from the beta that clustering is on by default.
Anyways, I can't tell you what exactly is missing, but the solution should be simple:
By default, clustering is not activated in JBoss. However, in the $JBOSS_HOME/server directory, you find three configuration folders: 'all', 'default', and 'minimal'. As you can imagine, by default the 'default' config is used.
There are two ways to switch on clustering:
a) start jboss with a command line switch to use the 'all' config (unfortunately, I don't know the exact syntax), or
b) rename 'default' to 'nocluster' and 'all' to 'default'.
Now you should see the clustering option coming up.
Copy server/all/lib/javagroups20.jar to server/default/lib
This enables the cluster services in JBoss. But be aware that this does NOT enable httpsession clustering!
You need to deploy jbossha-httpsession.sar. This is not in the binary distribution. You have to compile JBoss yourself to get it. It is in cluster/output/lib after compilation.
... and don't forget to add cluster-service.xml and jboss-ha.jar! Javagroups.jar is necessary for clustering but doesn't activate jboss clustering by itself.