I do not know much about Eclipse, but according to this tutorial everything should work fine (I am using the .zip archive). The error seems to occur only when Android Development Tools are checked.
Here is the full error code:
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.