Perhaps there is a max-width set for the main column in the _ColumnsThree.cshtml template which prevents the Nivo from being displayed? Or maybe the _ColumnsThree.cshtml doesn't even reference the widgetzone which Nivo slider is configured to appear within?
@Html.Widget("home_page_top")
The Nivo Slider should appear in that widgetzone if you have it enabled. If you don't see it in a browser, view the code in the view to see if it is referenced. /Themes/YOURTHEME/Views/Home/Index.cshtml