Simply execute the install script from Terminal and pass the image to write. (Image, not ZIP)
eg. sudo ./install ~/Downloads/wheezyDebian.img
Select the disk to write the image to by selecting the disk number provided in the output.
Wait for disk to finish writing. You can check the write progress with Ctrl+T.
Make absolutely sure to select the correct disk from the list of mounted disks output by the script. Selecting your system drive WILL overwrite it!
If you are unsure which disk you need to select you can remove the SD card, check the mounted disks by running df -hl and
then re-check after re-inserting the SD card.
The disk name will likely be something similar to: /dev/disk2s1