When Microsoft released the Visual Studio 2012 Shell, support for building Windows Store apps (formerly known as Metro apps) was missing. This certainly frustrates Windows Store Development with Oxygene which ships with the Shell.
Fortunately I’ve found a work-around to this annoying omission. Simply install Visual Studio 2012 Express for Windows 8 (free download) and then install (or re-install) Oxygene for .NET. This activates the support for WinRT templates and the Modern UI designers in the Shell. After that you can use the WinRT and Modern UI designers in the Shell with Oxygene for .NET.
The Visual Studio Shell and the Express Editions share many resources. As long as the Express Edition is installed, the Visual Studio Shell has WinRT and Modern UI support available for use in Oxygene for .NET development. The Express edition does not support Oxygene, so you will need to continue using the Shell.
Update: It was reported that it may be necessary to uninstall the Shell if you have trouble installing the Express Edition. So if you run into trouble you might try that.