The wizard only shows types for which CAD Document templates exist and for which the type is instantiable. You probably just need to create some templates in Windchill that utilize your soft type so they'll show up in the list in the New CAD Doc wizard.
For example, I have two soft types Test CAD Dcoument Type and Test CAD Doc 2:
Test CAD Document Type has a template defined for a CAD Part, Test CAD Doc 2 has no templates defined. Both are marked as Instantiable in the Type and Attribute Management.
My New CAD Document Wizard lists Test CAD Document Type (along with all the others that have templates), but not Test Doc 2:
