Welcome to the Apple Support Communities
Snow Leopard is a paid upgrade, so you can't get it in Software Update.
First, check if your Mac is supported > http://support.apple.com/kb/sp575 Then, buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
After that, make a Time Machine backup and/or a clone with Carbon Copy Cloner, insert the DVD and upgrade. The disc will upgrade OS X to 10.6.3, so open > Software Update and install the most recent version