Attached is your project modified so that it now operates properly. Basically I manually modified the Circle class name back to Class1 and re-built the solution. In addition I added a C# console project to the VisualAPL solution to server as an illustrative test harness. This console project shows how the VisualAPL .Net assembly can be used by C# or any other .Net language.
To change the default Class1 class name to Circle, follow the instruction at: http://forum.apl2000.com/viewtopic.php?t=453
I have also attached a pdf-format document which shows various elements of the working project.
Keep those questions coming!!!