To install the
preview
plugin, please use the Update mechanism from within Eclipse and provide it with the following URL:
https://dl-ssl.google.com/android/eclipse-preview/
Instructions are available on the
ADT Plugin For Eclipse
page on the
android.com
web site.
If you can't use the Eclipse update-site, you can find the latest
ADT-21.1.0_rc3.zip
here.