This is just a failsafe mechanism of Telerik OpenAccess ORM. It is there to prevent you from enabling a project that you want to add the Domain Model to. This is due to the fact that the Domain Model Designer cannot work in an enabled project.
What you need to do to add the visual designer is right click on the project, choose Add New Item, find the Telerik OpenAccess Domain Model template and add it to the project.
Hope this helps.
the Telerik team
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