If you face this error message while adding a reference to your project in Visual Studio try the following steps.
- In File Explorer go to the directory of the DLL you are trying to add as reference.
- Right Click and select properties
- Click on UnBlock Button
- Go to Visual Studio and again try to add the file as reference. Now you shouldn’t see this issue.
Note: I faced the error message “A reference to a higher version or incompatible assembly cannot be added to the project” while trying to add AdMob DLL in to my Windows Phone project.
A reference to a higher version or incompatible assembly cannot be added to the project
http://vkslabs.com/reference-higher-version-incompatible-assembly-added-project/