It looks like you will need to restart your SQL Server according to the MSDN link on enabling Mixed-Mode Authentication
There is another article (user comment at the bottom) referencing a registry key that you can look at; it also says you will need to restart the service once you have changed it.
From the first link:
To change security authentication mode:
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In SQL Server Management Studio Object Explorer, right-click the server, and then click Properties.
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On the Security page, under Server authentication, select the new server authentication mode, and then click OK.
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In the SQL Server Management Studio dialog box, click OK to acknowledge the requirement to restart SQL Server.
To restart SQL Server from SQL Server Management Studio
- In Object Explorer, right-click your server, and then click Restart. If SQL Server Agent is running, it must also be restarted.