The Icenium extension for Visual Studio is still in Beta and in this version, you cannot access any existing projects created with Icenium Mist or Graphite i.e. switching between Graphite and the VS extension is not a supported scenario. Our friends from Falafel have prepared a short tutorial how you might go about manually port a project from Graphite to the VS extension, however keep in mind that if you port your Mist/Graphite project to Visual Studio, the following limitations will apply:
You will not be using the Icenium cloud storage and version control and your changes in Visual Studio will not be synced to your Mist/Graphite project. You will need to manually sync any changes.