ObjectARX 2009 SDK had a nice sample project called "SimpleSquare" that demonstrated the managed wrapper creation that allows access to the custom entity from a managed code. I have migrated this sample project to work with AutoCAD 2013 and Visual Studio 2010.
The changes are mainly to conform to the new CLR syntax as the original sample used the old CLR syntax.
Here is the sample project for download.
To try this, place the "SimpleSquare" folder under "<ObjectARX 2013 SDK folder>\samples\entity" and follow the steps explained in the ReadMe.txt