What this almost certainly means is that your application is being compiled as 64-bit, whereas the code in OpenCV is 32-bit, or vice-versa.
In Visual Sudio:
- Right click on your project in Solution Explorer and choose "Properties"
- Choose the "Build" tab in the project properties that open
- Look at the "Platform Target" option about a quarter of the way down
- Set it to
x86 if it's currently set to x64 or Any CPU
Now try re-running your application, the error should no longer occur.
Note: The download page for OpenCV states that "It does not contain 64-bit binaries"