ADT 11 focuses on editor improvements. First, it offers several new visual refactoring operations, such as “Extract Include” and “Extract Style,” which help automatically extract duplicated layout fragments and style attributes into reusable layouts, styles, and themes.
Second, the visual layout editor now supports fragments, palette configurations, and improved support for custom views.
Last, XML editing has been improved with new quick fixes, code completion in more file types and many “go to declaration” enhancements.
ADT 11 packs a long list of new features and enhancements. Visit our ADT page for more details.
Source: Android Developers Blog - New Editing Features in Eclipse plug-in for Android
If you have already installed Android SDK, ADT for Eclipse, you can update ADT in Eclipse:
Eclipse Menu > Help > Check for Updates >
After ADT updated, may be you have to re-assign your SdK folder:
Eclipse Menu > Window > Preferences > Select Android on the left, and Browse to select your SDK Location.