When you create a matrix field on your form definition and you specify a table for field validation, Show candidates in Verify is disabled. This is because the Show candidates in Verify option and the grid verifier are incompatible. In order to select this option, you must deselect Use grid verifier.
Show candidates in Verify cannot be used with user-defined views created using FORMS’ Verify view designer. No drop-down table is displayed in the Field dialog.