you can access these using Arrowhead1End and Arrowhead2End which are properties of LinearDimension geometries. Both can be found here. Below is an example, note that the points returned are 2d points in the dimension plane:
Thank you for the reply. I have now incorporated it into my script and it is working perfectly.
I knew that I need to use Rhino common to access those properties, but I did not know how to cast the object into linear dimension object and how to access its properties. Both the arrowheadEnds and Plane.PointAt properties are very useful for me.