Symptoms:
Explorer Toolbar Visible checkbox option is disabled.  How do I enable it?  Or, How do prevent the Sperry Software tab from being shown in Outlook 2010 and higher?

Cause: 
This is by design.

Resolution:
The Explorer Toolbar Visible checkbox option is only enabled on Outlook 2007, because on Outlook 2010, the Sperry Software tab can be customized by going to File…Options…Customize the ribbon.  Uncheck the "Sperry Software" tab on the right.