You can add new fields by navigating to the Layout tab and clicking the small + symbol in any column. You can either add an existing field from the drop-down list, or you can create a new one. Remember, when you create a new field, you will still need to put the details in the Details Tab before it becomes visible in the signature.
When creating a new field, you will be asked to name the new field. The name is only an identifier so you can find that field in the Details Tab and modify it later. You will also be asked what kind of field it is so you can get the correct adjustment options for it in the Details Tab.