SOLIDWORKS add-ins are in-process applications hosted within the sldworks.exe process.
When debugging SOLIDWORKS add-ins it is recommended to specify the full path to the SOLIDWORKS executable for the Start external program option in the project settings under the Debug tab.
In this case it is possible to launch SOLIDWORKS and attach to the process automatically directly form Visual Studio by calling Start command or clicking F5
To attach to the running SOLIDWORKS instance use the Debug->Attach To Process... command
and select SLDWORKS.exe process form the list
In addition, xCAD.NET framework allows to debug original source code directly from the nuge package via Source Link. See Debugging xCAD.NET Source Code article for more information of how to enable Source Link.