Woo hoo! I finally worked it out after reading a few OSX howtos

Running the flasher utility with sudo doesn't quite cut it, and yet I was a bit reluctant to open up the security issues created by having a root account on the machine.

What you need to do, is open a root shell with sudo -s and run the flasher utility from there... No need to enable root!

users-Computer:~ user$ sudo -s
Password:
users-Computer:~ root# ls
.DS_Store .Xauthority .fonts.cache-1 .ssh Desktop Library Music
.Trash .bash_history .gimp-2.2 .xinitrc.old Documents Maemo tools Pictures
users-Computer:~ root# cd Maemo\ tools/
users-Computer:~/Maemo tools root# ls
.DS_Store flasher
users-Computer:~/Maemo tools root# ./flasher --enable-rd-mode --reboot
Suitable USB device not found, waiting
USB device found found at bus 001, device address 004-0421-0105-02-00
Found board Nokia 770 (F5)
NOLO version 0.9.0
The device is now in R&D mode
users-Computer:~/Maemo tools root#