Disculpa el retraso.
Objectarx es C++, no visual Basic
Si descargas el sdk del arx en su version completa veras toda la documentacion disponible que viene con el. Luego si quieres hacerlo en visual basic olvidate del objectarx, y consulta la ayuda del autocad y entra en el foro de VBA de www.hispacad.com. Ahi de fijo habra quien te oriente
Sobre la ayuda del autocad te adjunto el fragmento donde mencionan las librerias a utilizar con .NET, esto por si te sirve de referencia de por donde buscar:
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To fully access AutoCAD automation objects from Microsoft Visual Studio® .NET, create references to the following files:
The AutoCAD 2009 type library, acax17enu.tlb, located at c:\program files\common files\autodesk shared.
The AutoCAD/ObjectDBX Common 17.0 type library, axdb17enu.tlb, located at c:\program files\common files\autodesk shared.
These references will make available the following primary interop assemblies: Autodesk.AutoCAD.Interop.dll (for AutoCAD-specific types), and Autodesk.AutoCAD.Interop.Common.dll (for types shared by ObjectDBXTM host applications). The interop assemblies are located in the global assembly cache; they map automation objects to .NET counterparts.
After you reference the type libraries, you can declare AutoCAD-based variables in Microsoft Visual Studio .NET, as in the following examples:
Dim objAcad As Autodesk.AutoCAD.Interop.AcadApplication
Dim objLine As Autodesk.AutoCAD.Interop.Common.AcadLine
You can load a .NET application using the NETLOAD command in AutoCAD.
Additional information about using a .NET application with AutoCAD is available from the Developer Center section of the Autodesk website.
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Saludos