Indeed, it is possible to see the Edit Style button disabled in Blend. We think this is more like VS / Blend issue than one related to issues in our code.
However, could you please try the following steps in order to resolve this:
- Clean and Rebuild the VS project
- Open the Blend via right click , open in Blend on the project file, not on the solution file.
- Perform the steps described in our documentation.
If you are using VS 2012 you can also use the Designer in VS , right click on the RadTreeViewItem, Edit Template Button - Edit Style.