Distroshare creates an installable Live ISO from an installed Ubuntu or derivative distribution.
It is a bash script, similar to Remastersys and its forks. The script is based on this tutorial: https://help.ubuntu.com/community/MakeALiveCD/DVD/BootableFlashFromHarddiskInstall.
To run the script, run it from the directory where it is located. For example:
cd ~/distroshare ./distroshare.sh
To boot the ISO from a USB stick, you can use the dd command like this:
dd if=isoimage.iso of=/dev/sdb bs=1M
where sdb is your USB drive. You should be able something similar on Mac OS X. You can also use UNetbootin: http://unetbootin.sourceforge.net/ to create a bootable USB drive.
Ubuntu Startup Disk creator won't be able to turn the iso into a bootable usb drive since Distroshare Ubuntu Imager uses grub2 as the bootloader.