If you are using C# 2008, here is one way to debug:
right click on your project in the "solution explorer", choose "Properties", on the popup window, find "Debug", then set "Start extenal program" with your ArcMap.exe (or ArcCatlog.exe), for Arc 10 is: C:\Program Files\ArcGIS\Desktop10.0\Bin\ArcMap.exe.
Then, you should be ready to debug from C#.