is Rhino3dmIO the right toolkit? As the name suggests, this is the toolkit to support 3dm file IO in third-party applications. RhinoCommon, on the other hand, is for plug-in creation. They use a shared codebase, but diverge in the amount of available functionality (all Rhino for plug-ins, file read/write for 3dmIO).
OnXForm is called simply Transform, but not all types have a mapping from Rhino.Net to RhinoCommon/Rhino3dmIO. For ArrayOnPoint3d, you can just use a more standard array of Point3d, (Point3d C#, Point3d() Vb.Net) or the specialized Point3dList class.
I hope this helps,
for Robert McNeel & Associates