The compatibility with the final release of Visual Studio 2012 was introduced in the more recent releases of Telerik OpenAccess ORM - Q2 2012 SP2 and later.
Our recommendation would be to migrate to the latest release of OpenAccess - Q3 2012 SP1 or the one we are about to release next week - Q1 2013. Using them you should be able to work with Visual Studio 2012 without any issues.
I hope that helps.
All the best,
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.