I downloaded a performance enhancing mod which requires me replace the game's d3d9.dll file, but I haven't been able to find this. In my DX10 folder, there are lots of folders with titles like "Apr2006_d3d9_30_x86" but I have no idea which, if any, of these folders would contain the file I need to replace. Can anybody offer some suggestions?

The author must be mistaken, there is no d3d9.dll file in the Skyrim folder by default. But that's where it has to go.