Ok, first of all, you have to have installed the following dependences, only install the last versions of: MS Visual C++ Runtime 2010 SP1 x64, Ms .NET Framework 4 and DirectX from Microsoft web site.
If you have them already installed; then reinstall them, remove the previous and install again, don't reinstall over the old ones, except DirectX (because it don't let you to uninstall).
Only if reinstalling didn't work; decompress the x64 version of xinput1_3.dll from the DirectX installer folder, and place it in Dolphin x64 folder.
I highly recommend the x64 version of Dolphin, is by far faster than the x86 counterparts.
Sorry for my English.