I am trying to install the ADT plugin for Eclipse. However, after I have went to \"Install New Software...\" and entered the plugin location https://dl-ssl.google.com/android/ec
Goto Preferences->Network Connections and set the correct option (native/direct as per your proxy settings on your network) restart eclipse and try again. [Switching to direct worked for me at home, however in office i had to configure it to native..] (Proxy settings require restart of eclipse IDE on some machines to take effect)
Ensure that your eclipse has all android toolkit dependencies (wst/emf...) installed (http://developer.android.com/resources/faq/troubleshooting.html#installeclipsecomponents)