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Did you buy a physical copy from a store?  You will need to discuss it with them.  Double check all packaging to make sure you didn't miss it.  It might be printed on the game manual or on another piece of paper in the box.

If you bought it from Origin, the game automatically is tied to your account.  It is already activated.  If it isn't showing up, make sure your on "My Games" tab and "All Games" are selected.

You may also try to fully close Origin and Relaunch.