Q: how do I add a device to my apple id?

If you mean register it to your support profile : https://supportprofile.apple.com/

If you mean so that it shows in the 'manage devices' section of your account then use iTunes Match, automatic downloads or redownload past purchases on the device : iTunes Store: Associating a device or computer to your Apple ID

You can log into your account on it via Settings > iTunes & App Store