Ok, so did you check that the fax you added to the "Printers & Fax" is working ok? does it shows Idle?
As far as I know all you have to do once you checked the fax status is to click print.... then PDF & Fax and finally type the phone number in the "To" field..... you shouldn't need to enter anything else... so you can safely choose to send fax after setting the phone number...
Please let me know...