The key thing with the DDG App is that you have completed the two settings it needs, you can access settings by clicking the "Settings" button on the application's main page.
You need to enter the name or IP address of your Global Catalog Server using its internal name. So if your server is call MainServer and your internal domain name is xyz.local then you should enter:
The other thing to enter is the internal domain name, so using the above example that would be:
If you're still having problems please contact Siv on 01886-888942