title: 'required items could not be found' error when installing ADT plugin
body: <blockquote>
    <p>Cannot complete the install because one or more required items could not be found.
      Software being installed: Android Development Tools 12.0.0.v201106281929-138431 (com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431)
      Missing requirement: Android Development Tools 12.0.0.v201106281929-138431 (com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431) requires 'org.eclipse.gef 0.0.0' but it could not be found</p>
  </blockquote>
  
  <p>Some info:</p>
  
  <ol>
  <li>Eclipse SDK Version: 3.6.2</li>
  <li>jdk-6u26-windows-i586</li>
  <li>Windows 7</li>
  </ol>
  
  <p>Any suggestions on how to solve this 'required items could not be found' error?</p>