The debugger starts from the Dynamics NAV Development Environment. The user with which you are logged in must be assigned as a user in SQL Server. Go to Tools | Debugger | Debug Session.
The Session List page will open, as shown in the following screenshot:
Note that the page shows all sessions on the current database from all companies.
The session you select can be any of the following:
Place the cursor on the line corresponding to the session you want to debug ...