Supermetrics are only viewable in the Custom UI. You can view these supermetrics using any of the available widgets that displays metrics in addition to the built-in "show detail" and drill-down dashboards. So you can use widgets to view them or the built-in dashboards to drill down in to them using "all attributes" views. SMs won't automatically show somewhere - like all other attributes they need to be selected for viewing in widgets.
Like you select attributes in a widget to show metrics, you will select SMs from the attributes list. SMs are always populated and ordered to be at the top of attributes lists.