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You are using Mac OS X 10.6.8. iCloud requires OS X 10.7.2 or later, so you have to upgrade OS X.
First, check that your computer is supported > http://www.apple.com/osx/specs If it's compatible, make a backup of your data with Time Machine and/or Carbon Copy Cloner, and check that your applications are supported > http://www.roaringapps.com
After doing all of these steps, open the App Store and purchase OS X Mountain Lion. When the download finishes, the Mountain Lion installer will show up, so follow the steps to upgrade to OS X Mountain Lion. After upgrading, open System Preferences > iCloud, and follow its steps.
If your Mac isn't compatible with OS X Mountain Lion, you can see if it's compatible with OS X Lion > http://support.apple.com/kb/HT4949 If it's compatible, call Apple to purchase OS X Lion > http://support.apple.com/kb/HE57