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Snow Leopard is a paid upgrade, so you can't get it in Software Update.
First, check if your computer 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 DVD will upgrade Mac OS X to 10.6.3, so open Apple menu > Software Update, and install 10.6.8. You may need to run Software Update again in order to install iTunes 11