If you're not up to the lastest Firmware on your 3DS you can't access Nintendo servers to redownload, especially in game.
You should have an EmuNand set up and you could update that to latest Console FW, and it would work. If you already have an Emunand, there's no reason not to update it to gain eShop access.
In addition it should not be asking you to redownload it in the first place. Maybe you got a bad DLC download. Try a different source or redownload.