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Before iOS 5, the predecesor to iCloud was MobileMe. Go to Mail, Contacts, Calendars in Settings. Add an Account, MobileMe, enter Apple ID and password, then turn Find my iPhone on.
"Mail" reports, "Cannot Get Mail. The user name or password for '.Mac Account' is incorrect."
As I recall, ".Mac" cost money, which we chose not to spend.
What next?
It shouldn't cost money for the Find my iPhone service, when you set it up, did it give you the option to turn Find my iPhone on?
Find My Phone was never available for the iPhone 3G. Then only way to add a 3G iPhone is to create a free Find My iPhone account on any iPhone 4, iPad, or iPod touch (4th generation) running iOS 4.2. Once you create an account on a qualifying device, use your Apple ID and password to enable Find My iPhone on your other devices running iOS 4.2.

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