Seems obvious right? But I couldn’t find any documentation. I figured I’d need an identity (“account/profile”) and I’d have to associate a credit card, and I knew each xbox user had a local identity and an optional xbox live/microsoft identity, but that’s about as far as it went. I didn’t know if purchases were associated with a console or an identity; Microsoft’s DRM docs suggested purchases went with a profile - which is wrong for skyrim add-ons.
Briefly, this is what I did. I’m sure there are ways to do it all from the Console, but if you try that be sure you connect a USB keyboard first. Trying to do data entry from the xbox controller will drive an old person (> 18y) mad. Also, if you get d0000034 when you try to buy add-ons from within Skyrim it’s just Microsoft’s brilliant way of telling you that you don’t have an XBOX/Live account/profile.
- Go to https://account.xbox.com and login if you have a Microsoft ID (I have one from Passport/Hotmail days) . If you don’t, create one.
- Add your credit card information and then buy your skyrim add-on purchases. In another purchase I had to buy a "Game", that required me to click a download to 360 button after purchase.
- On your xbox console login with the profile you created, you'll want to plug in a USB keyboard to make entering your password less painful. Press the xbox control silver central button to see profile associated menu that shows downloads. The downloads window can take a long time to appear (shows empty white screen) and downloads may be slow.
- After the download completes it should be available to all users of the console. I’m not sure what happens if you are logged into more than one console at the same time. This is the language skyrim shows at purchase time:
"The item you are buying is subject to usage restrictions. You can use this item on the first Xbox 360 console that you download it with. Access to this item will also be granted to all users on this first console. If you transfer the item using a memory unit or other storage device, you will also be able to use it on other Xbox 360 consoles, but you’ll need to sign in to Xbox Live with your Xbox Live account on that console before accessing the item."
 If you had a profile on the xbox already associated with your Microsoft ID, you’ll see that here. However, if you decide to use this UI to change your gamertag, the profile on the xbox will lose its relationship to the xbox profile. Evidently the ‘key’ is the gamertag rather than the Microsoft account. Yeah, Microsoft is just like it always was.