Cannot complete the install because one or more required items could not be found. Software being installed: Android Development Tools 12.0.0.v20110628192
Have had the same problem a few times. You need to add the following urls to the "Available Software Sites" of your eclipse installation:
That should fix it.
If not, see my install paths, and add as needed:
Also, make sure you tick the box on install that says "Contact all update sites during install to find required software".
Use the Android Bundle http://developer.android.com/sdk/index.html#download
(see my answer here https://stackoverflow.com/a/14754583/414969)