/AI (Specify Metadata Directories)
Specifies a directory that the compiler will search to resolve file references passed to the #using directive.
Only one directory can be passed to an /AI invocation. Specify one /AI option for each path you want the compiler to search.
To set this compiler option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.
Click the C/C++ folder.
Click the General property page.
Modify the Resolve #using References property.