Adding Netbeans API source navigation to your Netbeans IDE
It took a while to find this entry describing how to include the Netbeans APIs source code. In short, you have to download the actual sources (the link in Geertjan’s Weblog is outdated) at http://download.netbeans.org/netbeans/6.0/final/zip/ (file netbeans-*-platform-src.zip) and then link that zip file to the platform in the platform manager. Make sure that the path doesn’t contains whitespaces (it didn’t work for me). To access the Netbeans Platform Manager just do Tools -> Netbeans Platforms ...