Can you please share with me which approach of working with OpenAccess are you using? If you are using the old approach than I believe that you have some configuration entries missing in one of your project.
In the project that is referencing your model you will need to add something like this in the app.config file:
This should fix your problem.
All the best,
the Telerik team
Q1’11 SP1 of Telerik OpenAccess is available for download
; also available is the Q2'11 Roadmap
for Telerik OpenAccess ORM.