Note to self:
The Delphi IDE stores a list of disabled packages (that is packages where the user removed the check mark in front of the package under Components -> Packages) in the registry under HKCU\<basekey>disabled packages.
<basekey> is the base key of the Delphi configuration, e.g.
Software\Embarcadero\BDS\9.0
So the disabled packages for Delphi XE2 are stored in
HKCU\Software\Embarcadero\BDS\9.0\disabled packages
If you remove the package from the list of known packages (stored in the registry under Known Packages) and later on add them again, these packages might still be disabled and sometimes setting the check mark in the dialog does not enable them. To fix that, just delete these packages from the disabled packages list using the Registry Editor.