The type or namespace name 'DirectX' does not exist in the class or namespace 'Microsoft'...

Oh, yes, it does.

If your previously working Managed DX code or samples stop working due to a missing reference, it's likely because a windows update with a newer DX end user runtime removed the SDK installed Managed DX dll.

Apparently the DX 9.0a end user runtime removed the Managed DX dll installed by the DX 9.0 SDK. This was probably fixed in the 9.0b end user runtime release. This wouldn't affect end user applications that ran on Managed DX, but it would prevent your Managed DX code from compiling.

Solution:
1. Download the DirectX 9.0b Redistributable for Software Developers. Yep, it's a 35MB download.
2. Install with the /installmanageddx setting - "dxdevrtm.exe /installmanageddx"

If you've installed the DX SDK after the 9.0a end user runtime release this isn't a problem for you. Come to think of it, if you've never installed the DX SDK this probably isn't a problem for you.