Custom lists don't appear in the Select from List window for form fields
Each custom list that you create is numbered 1, 2 or 3, based on the field in which you entered the list name in the Custom List and Field Names window. The form fields that correspond to each custom list are identified by the list numbers, not by their names, in the Select from List window. (Adding custom lists and fields)
For step-by-step procedures for adding a custom list data field to a form, see To add data fields to a form.