Cant run 64-bit dlls (native plugin)

Are you sure you read this page carefully?

To sum up the most important points:

• make sure your plugin exports it's method in an extern "C" { } block

• Place your DLL file inside "/Assets/Plugins/"

• If you build a x86_64 plugin make sure you have selected 64 bit build in the build settings

• If you want to build both 32 and 64 bit builds you have to compile two seperate versions of your plugin and place them in the appropriate sub folder of the plugins folder ( "/x86/" and "/x86_64/" )

• Finally, just in case it's not obvious: You can't use native code plugins in webplayer or WebGL builds for security reasons.

edit
I just created a simple "plugin" like this in C++:

 extern "C"
 {
     int __declspec(dllexport) DoSomething(int a, int b)
     {
         return a * b ;
     }
 }

And use it in Unity like this:

 [DllImport("UnityTestPlugin")]
 private static extern int DoSomething(int a, int b);
 
 void Start ()
 {
     Debug.Log("Result : " + DoSomething(3, 5));
 }

And it prints "Result :15" just fine. Of course i set the target platform in VS to "x64" and in Unity i set the target platform to Windows Standalone 64 bit.

Also check the inspector when you select your dll in Unity. Now you can select for which platform and which architecture a DLL should be used. By default a DLL is used for all platforms and all architectures. However since it's a x64 DLL it makes no sense to use it in x86 builds.