To be able to do Navigate ⇒ Go to Source on netbeans classes like TopComponent you need to link the sources to the Netbeans Plaform using the Netbeans Plaform Manager as explained in Hyperlinking to NetBeans API Sources.
Be sure to download the proper source zip from the netbeans download page, otherwise the Platform Manager won’t let you attach the source file to the platform (all invalid options are greyed out/dimmed):
- Netbeans 6.5 platform src zip. netbeans-6.5.1-200903060201-platform-src.zip and not netbeans-6.5.1-200903060201-src.zip from netbeans zips download page.
- Netbeans 6.7beta. Select netbeans-6.7_m3-200904011705-platform-src.zip and NOT netbeans-6.7_m3-200904011705-src.zip from http://bits.netbeans.org/netbeans/6.7/m3/zip.html