Thank you for the feedback.
Indeed, in version Q1 2013 of OpenAccess ORM dragging-and-dropping interfaces from Model Object Explorer on the surface of Visual Designer is not possible. It is in our TODO list, however, and is scheduled for release with our Q2 2013 version.
Currently, I would suggest to you the following workaround:
1) Open the diagram that holds the interface
2) Right-click on the interface and select Organize Classes / Move To New Diagram
3) Enhance the newly created digram as needed
4) Save the .rlinq file
I hope you find that feasible. If you have additional questions, do not hesitate to get back to us.
the Telerik team