Security Manager 6.x
NetIQ Change Guardian for Active Directory 1.8
Browsing to add a user will not work with CGAD and domains with trust issues. You can however input the text in the field itself without browsing. Type the user in this format:
Please note that the GUI will not validate the user that you input, so you will need to test this after adding the user. Also, you can technically go into the Development Console and add the user there, however you will need to modify a much larger set of rules. It is advisable to only edit this in the Configuration Wizard.