Restore the Snow Leopard .dmg to the flash drive using Disk Utility, then you can boot from it.
Open Disk Utility and drag the .dmg file directly to the white field on the left of the Disk Utility window.
Highlight the .dmg file in the list and click Restore. It will have a field for a destination. You must have a thumb-drive or external hard-drive partition formatted in the GUID partition map.
From the list on the left, drag the drive or partition that you wish to use as a Snow Leopard drive to the Destination field.
All of the data on the partition or drive will be erased and replaced.
Click the Restore button below, enter your Admin password if asked, and the process will begin.