What url do you use for it because http://www.red5.org/projects/red5plugin/update_0.1.x/ gots redirected to http://code.google.com/p/red5/, and my eclipse is heavily complaining about network connection problems (other plugins update correctly).
Go here for info on the plugin: http://www.red5.org/red5-ide-plugin/
Eclipse 3.2+ Link: http://www.red5.org/downloads/projects/red5plugin/update_0.2.x/