Which plugin did you add? Did you check the compatibility of the plugin with the version of Eclipse you are using? Did you follow installation instructions, and restart eclipse after installing the plugin? Also can you provide with some more details of when are you getting the error and the xact message in the error? I assume you already have a weblogic domain and server created, and are trying to add the server to eclipse.
He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever - Chinese proverb