I think you need more than one physical server host, with more than one weblogic server instance across them in your weblogic domain, to make a "cluster" in weblogic? at least you did in weblogic 5.1. Anyway, whatever, I think if you have the prerequisite server instances up and running in your domain, you just use the admin console (running the admin web app on one of the servers) to create a cluster in your domain, assign servers from your domain to the cluster, etc.