Any reference-type property for which the base definition type has no reference will appear blank:

You have to edit the base definition (possible through the Instance Of property) and set a reference in the same property and press OK.

If this was done without closing the Instance definition, press Apply to force it to update.

Now the property looks like this:

To add a new reference, right-click in a blank cell in the Name column and press the Choices button that appears.

You can add only one instance reference per type reference stored in the base definition.