If you want to connect to a SQL instance behind a firewall, you must enable the SQL instance for communication. To enable the SQL instance for communication, you must make the SQL port static and configure the Windows Firewall.
The Backup Exec SQL instance is configured to use a dynamic port by default. Each time SQL Server is started, the port number can change.
See Changing the dynamic port on the SQL Express instance in CASO to a static port.
See Opening a SQL port in CASO for a SQL 2005 or 2008 instance.
You also must configure the Windows Firewall to allow connections to the SQL instance. There may be multiple ways to configure the Windows Firewall based on your system configuration. You can add sqlsvr.exe and sqlbrowser.exe to the Windows Firewall Exceptions list or you can open a port in the Windows Firewall for TCP access. Refer to the Microsoft Knowledge Base for more information or to determine which configuration is best for your network.