...
Go to the Field List to add the Line Item field.
Module → Module Name → Forms(tab) → Create/Update Fields… (button)Select the Add Field button in the top right.
Type in an Internal Name and System Name then from the Type drop down select Line Item.
Save the Field List at the bottom.
Go to the Default Form to add the Line Item to the form.
Module → Module Name → Forms(tab) → Default (name link)Drag and drop the Line Item field into the form fields above the section that is intended to repeat.
Drag and drop a Section End below where the form fields section intended to repeat should stop.
Save the Default Form.
Visit the main grid and validate the changes were applied successfully by viewing the form.
...