That's because Snow Leopard isn't compatible with Snow Leopard. In your case, on Snow Leopard you can only use your iCloud mail > http://support.apple.com/kb/HT5330
If you want to use all iCloud features on your MacBook Pro, you have to upgrade to OS X Lion or Mountain Lion. First, see if your computer is supported > http://www.apple.com/osx/specs
If it's compatible, make a Time Machine backup and/or a clone with Carbon Copy Cloner, open App Store and purchase Mountain Lion. When the download finishes, the installer will show up, so follow the steps to upgrade to Mountain Lion. After upgrading, iCloud will show up on System Preferences.
Also, check if your apps are supported > http://www.roaringapps.com