Brother MFC-3420C - Manual (Page 164)

9 - 9 USING YOUR MACHINE WITH A MACINTOSH ®
4
Select Facsimile from the Output pull-down menu.
5
To address the fax, type a fax number in the Input Fax Number
box, and the click Print.
If you want to send a fax to more than one number, click Add
after entering the first fax number. The destination fax numbers
will be listed in the dialog box.