Try to manually install the Netbeans plugin from here: http://dlc.sun.com.edgesuite.net/netbeans/updates/6.9/uc/m1/dev/modules/enterprise/org-netbeans-modules-j2ee-sun-appsrv81.nbm
Download that file locally and then from NB:
Tools-Plugins-Downloaded - Add Plugins
That fixed it for my NB 7.3.1.