OSX 10.6 Snow Leopard is a paid upgrade and only avaiable as an Install DVD, not for download.
Make sure your MacBook meets the system requirements for OSX Snow Leopard:
- Mac computer with an Intel processor
- 1GB of memory
- 5GB of available disk space
- DVD drive for installation
- Some features require a compatible Internet service provider; fees may apply.
- Some features require Apple's MobileMeservice; fees and terms apply.
After the installation from the DVD you can then use Software Update to download and install the latest system update (10.6.8).