JBoss AS 5.1 is not supported in that version of Tools.
For 5.1 support, you need the latest JBoss Tools 3.1.0.M1, which requires Eclipse 3.5RC3 or later:
Update site: http://download.jboss.org/jbosstools/updates/development/
note, 5.1 support is planned to be available in a 3.0.x maintenance release.
Great! Anyway, current release work for me fine.
for JBT 3.0.1 ? AS 4.x or AS 5.0
But you should be able to use 5.1 if you just set the ports manually (the files were changed in 5.1 hence our code won't find it)
Or try using the latest nightly build of 3.0.x from http://download.jboss.org/jbosstools/builds/nightly/3.0.2.GA/
It should work in there - if not let us know :)