@Brenden I just tried creating a new Document Type to try to avoid using 'Custom' in the Namespace.
This time I created a Document Type with Namespace=MyNamespace and Name=MyDocumentType.
Looking in the CMS_Class table the ClassName=MyNamespace.MyDocumentType.
In the Uniselector I set the Object type=MyNamespace.MyDocumentType and I get the same error as before;
Object type 'mynamespace.mydocumenttype' not found.
As a work around I am using the List box control, ticked Allow multiple choices and am using the following SQL Query for the Data Source;
SELECT CourseID, Name FROM dbo.View_custom_Course_Joined ORDER BY NodeOrder
This workaround does meet the requirement of selecting multiple Documents. But in v7 our CMS Editors preferred to use the Uniselector.
Cheers,
Darren