/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. For example, to add both C:\Project\Meta and C:\Common\Meta to the compiler search path for #using directives, add /AI"C:\Project\Meta" /AI"C:\Common\Meta" to the compiler command line or add each directory to the Additional #using Directories property in Visual Studio.
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.
In the navigation pane, select Configuration Properties, C/C++, General.
Modify the Additional #using Directories property.