Q: New IPad not showing in managed devices

Have you downloaded anything by automatic downloads since you turned it 'on' ? Redownloading past purchases on it might also get it to show there.

Other people have posted that it doesn't always seem to work, though it should so.

iTunes Store: Associating a device or computer to your Apple ID :

When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID. This article describes how to manage these associations.