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Did you redeem the code back when you received it? If so, ODST will show up under “Manage Game and Add-Ons” —> “Ready to Install.”
Codes like that are single use OP, so one you redeemed it then it can’t be used again and there’s no way to resend it. If it was activated on your Xbox Live account then you should be able to find it using the method noted in the post above ^