Did you receive a code for Xbox or Playstation? Currently, New Light isn't available on those storefronts yet so the only way to gain access is through purchasing on the storefront and receiving it as a bonus, or entering a code if purchased from a different retailer. For PC, New Light is available for predownload now, so if you received a code for Steam, I don't know what to tell you. Either way, I would suggest using it just in case the wording on it doesn't match the contents, such as if it's actually the pre-order bonus code and that's how you gain the preorder bonuses once Shadowkeep launches.