For anyone struggling to use this on a Mac (until hopefully one day a MAC binary is made avalable), I found a way to run it without having to boot a virtual machine or go into bootcamp.
Download PlayOnMac ([Only registered and activated users can see links. ]) - it's a free download.
Launch the program and click on "configure"
On the new configuration screen, click "new"
This will launch the "Virtual drive creator"
Click next twice
Type in any name you want and click next
This will create a new virtual drive which appears in the list on the left of the configuration screen.
Now click on your new virtual drive
On the right section of the window, click on the "wine" tab.
Then click command prompt
This will launch a familiar windows CMD window.
Put the gimgunlock.exe file in the same folder as the .img file you wish to unlock
Navigate the command prompt to the folder where both the gimgunlock.exe file and the .img file are stored.
(navigate using the usual cd.. and cd commands as per windows)
Once in the correct directory (you can do a dir command to check), type gimgunlock.exe xxxxx.img (where xxxxx is the name of the img file)
You will then see the command prompt spool through a whole list of stuff.
Voila! IMG is unlocked.
I've yet to figure out how to "package" the exe into a standalone .app which supports drag'n'drop of a .img file onto it. Whoever manages that will be a star!