If you're looking for a namespace that begins with "Microsoft.DirectX" you won't find one. XNA isn't MDX. All the namespaces you need for XNA are under Microsoft.XNA. For example, what used to be the DirectX device is now the GraphicsDevice and is in the Microsoft.XNA.Framework.Graphics namespace, as are textures, surfaces, etc.

If you're using XNA, you DON'T want to be referencing any of the Managed DirectX assemblies. They don't play together at all. You can use DirectInput, but don't try to use the others.

Why do you think you need to reference anything in the Microsoft.DirectX... namespaces?

Well, as I said, using DirectInput is fine. I thought you were trying to use DirectX graphics, which won't work with XNA.

The "right" way is to add a reference to the Microsoft.DirectX.DirectInput assembly as you showed in your previous post. Just select Microsoft.DirectX.DirectInput. Then add "using Microsoft.DirectX.DirectInput;" to whatever source files will be making use of the namespace.