In order to set a Domain Controller as target server, please follow these steps:
- Select the Domain Controller desired to be a target server
- Click on the "Properties" button
- Click on the "Edit properties" button
- Enable the "User replication target" and "Computer replication target" checkboxes, then click on the "Save" button
- Click on the "Back" link next to the "Edit properties" button in order to go back to the Service Management list
The Domain Controller now should have the "Replicate" column value set to "Yes" and hence should be a target server.