Do you have a 64 bit OS? Need 64 bit OS to run directx11. Check out the requirements from the benchmark site:

If DirectX® 9.0c is not installed on your PC, please download and install it.
* Windows® 7 64-bit or higher is required to run the DirectX® 11 version, as are a compatible graphics card and the corresponding drivers. Please refer to your graphics card manual for further details.
* DirectX® 11 can be downloaded from the link provided above.