You need to load compiled version your addon in the archicad's addon manager ( do not copy it but load where it is located after compilation ).
Set path of your archicad.exe as executable for debug session.
It is:
- On the Debug Tab of the Project Settings for Visual Studio 6
- On Debugging panel, Command field of the project properties for Visual Studio NET
( or set it empty. Use browse to point the archiacd.exe when you will be asked at Debug start )
After that you can use usual Debug Start (Go) for debugging. Archicad will start.
Set breakpoint etc.