Every Microsoft Office InfoPath form (.xml file) is based on an underlying form template (.xsn file). A form template defines the overall appearance and behavior of the items in the form, such as text boxes and check boxes. If you want to design a form template instead of filling out a form, you must switch to design mode.
On the File menu, click Design a Form.
Note: If the Design a Form command is unavailable, the form designer may have "locked" the form to prevent unauthorized changes.
In the Design a Form dialog box, click the appropriate option.
When you have successfully switched to design mode, a (Design) prefix will appear in the InfoPath title bar.