I updated ADT through Eclipse today to version 21. Now when I open the SDK Manager I would expect there to be a folder like \"Android 4.2 (API 17)\", which would contain the SDK
I had this problem in ubuntu.
Open a terminal and navigate to the android-sdk-linux folder: cd android-sdk-linux
cd android-sdk-linux
Then manually update the sdk: tools/android update sdk -u
tools/android update sdk -u
After it is done, restart Eclipse.