An attempt was made to load a program with an incorrect format error normally means you are trying to load a 32 bit DLL from 64 bit code or vise-vera.
Your C++ project appears to be built as 64 bit code (since your path to the DLL includes a X64 in it). That woule likely mean that your C# application is running in 32 bit mode. That is likely as well since as of Visual Studio 2010 client application projects (console, WinForms, WPF) default to a 32 bit target.
Check the project settings of your C# application by double-clicking on the Properties item in the Solution Explorer. Then choose the "Build" section and ensure that the platform target is "x64". If it is "x86" that would mean that your C# code was running in 32 but mode.
Setting Enable 32-Bit Applications
setting in IIS is incorrect for two reasons. First your wanted your C# code to run in 64 bit mode, and enableing that setting would force the code to run in 32 bit mode. Secondly that setting only affects applications running in IIS (read ASP.Net) not client applications.