Fax Pro is a Mac program designed for people who work with faxes on a daily basis. The application brings you a straightforward interface, lets you send faxes to other people at reasonable prices, and use its services without paying a buck, for as long as you like.
Before using this utility on your Mac, you first need to create a personal account on its main window. The application will let you log in as soon as the new account is validated (you can use your default web browser to log in to the email account you provided for registration and validate the new Fax Pro account).
In order to send or receive any faxes you need to acquire a fax number. You can do that by making an in-app purchase. Afterwards, all you need to do is acquire credit and you can start sending and receiving faxes on your Mac.
According to multiple reviews that were left on this program's Mac App Store page, the utility has some problems regarding the payments for credit (for example, one user mentioned that he payed for a fax number and his request failed; yet money from his account were taken). In my testing, I wasn't able to acquire any credit, so I can't be 100% sure that these reviews are accurate. Personally, I prefer trusting the reviews of other Mac users instead of believing what's being said on the homepage of the developer. I tested other fax tools that lacked this type of user reviews and worked great on my Mac.
Hence, I'd say you should look for a different fax program for your Mac.