In certain installations, an AD object merge cannot be performed because the drop-down titled "Specify the Active Directory Attribute that matches:" is not populated. Without a selection, the next button is grayed out.
To resolve this issue please follow the steps below:
1. In Control Panel | Add Remove programs, un-install Notes Migrator for Exchange
2. Re-install the latest build of Notes Migrator 4.0
3. All existing database information should be retained and will be connected automatically upon the re-install.
After the above is performed, the drop-down in question will function properly and an AD Object Merge task can be run.