There are two ways you can go about debugging in Microsoft Dynamics NAV:

• Debugging the Classic client: By using a debugger built into the C/SIDE development environment of the Classic client, it is possible to debug business logic contained in all kinds of objects except pages.
• Debugging the Service Tier: To address the limitation of the Classic clients in respect to debugging page objects, you can use Microsoft Visual Studio to debug the Service Tier. Microsoft Dynamics NAV 2009 translates the C/AL code into C#, which is then compiled into .NET assemblies. By attaching Visual Studio to Microsoft Dynamics NAV Server service, it is possible to debug the business logic in the Service Tier.

(C) http://www.troubleshootingwiki.org/Debugging_Dynamic_NAV

Altro link interessante

Debugging in NAV 2009

(C) http://blogs.msdn.com/clausl/archive/2008/10/14/debugging-in-nav-2009.aspx