iCloud needs OS X 10.7.2 or later and you have Mac OS X 10.6.8. You have to upgrade to OS X Lion or Mountain Lion.
First, check if your Mac is supported > http://www.apple.com/osx/specs If it's supported, make a Time Machine backup and/or a clone with Carbon Copy Cloner, open App Store and purchase Mountain Lion. When its download finishes, the installer will appear, so follow the steps to upgrade Mac OS X. After upgrading, you will be able to log in iCloud on System Preferences > iCloud.
Check if your programmes are supported > http://www.roaringapps.com