Open MySQL Workbench and start the Migration Wizard 
From the main MySQL Workbench screen you can start the Migration Wizard by clicking on the Database Migration launcher in the Workbench Central panel or through Database –> Migrate in the main menu. 
A new tab showing the Overview page of the Migration Wizard should appear. 
Read carefully the Prerequisites section. You can read there that you need an ODBC driver for your source RDBMS installed.  Any recent version of Windows comes with some ODBC drivers installed. For Windows 2000 and earlier these can be installed with the Microsoft Data Access Components (MDAC).

You should check if you have an ODBC driver for SQL Server. Start the Windows ODBC Data Source Administrator from MySQL Workbench using the Plugins –> Start ODBC Administrator menu item or just open a Windows terminal and type odbcad32.exe. Once there, go to the Drivers tab. You should see something like this: