I have just installed Eclipse 3.5 for Java EE developers (\"Galileo\").
I need to add an extra plugin, but when I select the \"Install New Software ...\" menu item,
Start by checking you error log under Window -> Show View -> Error Log. See if you're getting any exception while trying to install new software.
Window -> Show View -> Error Log
You can also install by downloading the plugin and unzipping it in the dropins folder.