How to: Add or Remove References By Using the Reference Manager says:
In the desktop projects the Core tab doesn’t appear by default. You can add the Windows Runtime by opening the shortcut menu for the project node, choosing Unload Project, adding the following snippet, and re-opening the project (on the project node choose Reload Project). When you invoke the Reference Manager dialog box, the Core tab appears.
<PropertyGroup> <TargetPlatformVersion>8.0</TargetPlatformVersion> </PropertyGroup>
Make sure to check the Windows box on this tab. You should then be able to use WinRT elements.