As we all know, Flash Builder (earlier Flex Builder) is based on Eclipse RCP, lacks rich XML editing capabilities which are needed for almost all projects, except the Hello World! kinds. For those who have used Eclipse, would agree that the Eclipse platform has a feature-rich built-in XML editing capabilities. It is easy to use the same capabilities in Flash Builder as well.

To install the plugin, launch your Flash Builder and then choose Help –> Install New Software. Add the f ollowing location to the list of Available Software Sites, for Flash Builder 4.0, and click OK. Choose the site in the drop down, and wait for the list to populate. Once the list has populated, choose Eclipse XML Editors and Tools and click on Install. Restart your workbench, and, you are done.

Hope this helps.