If you have the 64-bit version of Windows 7, you'll have to use DOSBox. 7th Guest is a 16-bit program and those won't run on 64-bit unless you use an emulated environment like DOSBox. Whether you have the DOS version or the Windows 95 version of 7th Guest, it still contains 16-bit code.
You can find the 7th Guest beta DirectX patch with a Google search. One site that has it is
***Computerhope***. Note that they have both Sanitarium and 7th Guest patches on that webpage. The patch you want is called t7gwinnv.zip on that webpage. You'll probably also have to use Compatibility mode.