2008 MacBook Air Can't Boot OS X Snow Leopard USB Install Drive
It can't find the kernel files.  When you turn on your Mac make sure you  pressing and holding the the "Option" key then selecting the USB flash?

If you are still having difficulty, then the creation of the USB flash disk didn't go so well.  I am assuming that you have a good DMG image of the Snow Leopard DVD. 

You can't just "copy" or restore the image to the USB flash, you have to format and partition it first.  See How to Create a Bootable USB Drive With OS X Snow Leopard for a good tutorial on how to do just that.  

Once you are done, you should be able to boot from the USB flash and install OS X.