You don't mount it per se like a drive connected to the mac.
You use fugu to ftp/ssh in to your iPhone.
Go to your iPhone wi-fi setting and ge tthe ip for your phone.
Enter the IP username: root and password: dottie in to the dialog box and click connect. Give it 20 seconds or so for it to connect. Once it connecs you can browse the files on your phone just like browsing files on a computer.
You phone has to be hacked/open/apptapped before fugu will connect.