You should notice that Unity cannot check unmanaged DLLs in the editor; you’ll have to run the game to check whether the linkage has been successful or not. This is not the case with managed DLLs, which can be checked statically.
Sadly i can't run the app on a simulator because we are using a 3rd party library which can not run on the simulator: Vuforia Engine applications must be deployed to a device to run; they cannot be run in the iOS simulator. You must enroll in the iOS Developer program to deploy ...