If you are receiving an error regarding a DLL file while loading rFactor, you may not have the latest version of DirectX. Below are two possible solutions.
In your rFactor directory, find the Support\DirectX\Install folder and load DXSETUP.EXE. This will run the installer for the latest version of DirectX. Once this setup is complete, rFactor should load properly.
If you do not have the installer or have a problem finding it, then you can download the DirectX Installer from Microsoft’s website here.