Are you running Safari on a PC or on a Mac? We are not able to support Explorer Online on Safari on the PC at this time. While Safari on the Mac (v4 and v5) is supported by Silverlight, Safari on the PC is not.
Supported browsers on the PC include IE7, IE8, FireFox3, Chrome4
Supported browsers on the Mac include Safari4, Safari5, FireFox3
You can review the Silverlight system requirements here: http://www.microsoft.com/getsilverlight/get-started/install/default.aspx
Andy