This page hosts XInput DLLs. If you need d3dx9_xx.dll or d3dx10_xx.dll, you need the D3DX DLLs page.
Check the error message carefully, because the filenames are quite similar.
All the DLLs are compressed into ZIP archives to save bandwidth. To install a DLL, extract it from its archive, and place it in your C:\WINDOWS\SYSTEM32 folder (or equivalent). If your PC doesn’t know what to do with ZIP files, install the free archiving tool 7-zip.
These files are hosted here without permission from Microsoft, and will be removed if anyone important complains.