Unfortunatly the OpenXML SDK 2.0 and 2.5 rely on .Net Framework namespaces (e.g. System.Xaml) which are not available for Windows 8 Store apps. So at the moment there is no way to use the OpenXML SDK in WinRT. As an alternative one has to use classes like ZipArchive in namespace System.Compression (see e.g. http://lunarfrog.com/blog/2013/01/21/deploying-zipped-data/) for compressing and decompressing packaged .docx-files in combination with LinQToXML for reading, adding or updating content. It would be very nice if Microsoft could release a new version of the SDK compatible with WinRT. That would make things definitely much easier.