In the latest versions of Telerik OpenAccess the Telerik.OpenAccess.Query
namespace is no longer needed in order to use the Extent<T> method. The method is now part of the root Telerik.OpenAccess
Could you please tell us which of the OpenAccess approaches you are using - the "classic" Forward/Reverse mapping wizards or the new visual designer? If you are using the new designer, you will have to use the auto-generated OpenAccessContext class to manage persistent objects instead of an object scope. Please try to give some more details on what you are trying to achieve, so we can help you further.
All the best,
the Telerik team