To be able to migrate from Microsoft SQL Server, ensure the following:
The source SQL Server instance is running, and accepts TCP connections.
You know the IP and port of the source SQL server instance. If you will be migrating using a Microsoft ODBC driver for SQL Server (the default in Windows), you will need to know the host and the name of the SQL Server instance.
Make sure that the SQL Server is reachable from where you will be running MySQL Workbench. More specifically, check the firewall settings.
Make sure that the user account has proper privileges to the database that will be migrated.