Hi Manuel,
JBoss WS Tools do depend on some parts from other plugins (AS, JMX, some common stuff and maybe sth else) so the best way to correctly install it you should use update site (http://download.jboss.org/jbosstools/updates/JBossTools-3.1.1.GA/) and install the plugin from there. This way the installation proccess will take care of all required dependecies and you should be ready to start using it.
HTH,
--lj