Love your pronunciation of foreign words, you're one of the few people who actually knows what he's saying, and HOW he's saying it. Also with Latin on your previous video, like Pegasus, with plural Pegasi, nobody ever says that. Kudos to you!
You can purchase it in-game after reaching/unlocking the Dragon's Gate in the campaign. It's only a few missions in. For some reason, it only can be purchased this way.
One way to use your "credit card" to get the DLC, is to link your NNID (Nintendo Network ID) to your Nintendo account. Once linked, the monies will appear and ready to buy in-game. I think you may have to "link" your 3DS to your Nintendo Switch as well.
