To ensure that the communication between Remote Desktop Manager for Mac and the SQL Server database is encrypted, an extensive procedure must be followed on the SQL Server instance.
Please consult this Microsoft technet article that provides detailed instruction: Encrypting Connection so SQL Server (technet).
After it has been properly configured, the only modification to perform in Remote Desktop Manager is to set a property in the Advanced Settings of the data source.
1. Create or edit an SQL Server data source, access the Advanced tab and click the Advanced Settings button
SQL Server - Advanced tab
2. Set the Encrypt property value to true. Click on OK to validate.
SQL Server - Advanced Settings