To create a new form field, click the Add Form Field tool.
Like adding any other type of object, you drag out a box on the PDF where you want to add the form field.
Upon marking out the box, a dialog will appear prompting you to specify the form field name.
Enter a form field name. (You should try to avoid using the same form field name or else PDFtypewriter will rename your fields in order to avoid several form fields from having the same name.)
Click the OK button and the form field will be added to the PDF.