Is this a media install or from a download client ?
If it's from media (CD/DVD) I'd suggest a reinstall, you could always download direct X from MS but it will be bundled with the game and included in the installer.
If you downloaded the game, you might want to look for a way of verifying that the game downloaded correctly and none of the files are corrupted.
As I say you could download a Direct X package from MS and install it over the top but DX installs tend to be customised by the game devs, so I doubt this would solve the problem.
I think your best bet is to do a full reinstall if you can. Unless the game installer comes with a repair function (which I doubt but you never know).