The WinRT version of the WriteableBitmap doesn't provide a .Render method, Look into the RenderTargetBitmap class. See this recent blog post for more info:
http://kodierer.blogspot.de/2013/12/easy-render-writeablebitmapex-with.html
http://kodierer.blogspot.de/2013/12/easy-render-writeablebitmapex-with.html
- Rene