We understand that you cannot connect to the Slingbox using your Mac, this error message that you are getting seems to be caused by the computer and not the Slingbox. Have you tried to connect using a different computer (Mac/PC)?
We recommend you to install a different browser like Firefox to determine if the error persists and the issue is just with Safari. After the installation, you should be able to connect again accessing Watch at slingbox.com.
Hope this helps and let us know how that works.