View the output of Adam Machanic's sp_WhoIsActive I'm attempting to look at the XML output by Adam Mechanic's sp_WhoIsActive stored procedure.  

When I use a typical XML editor, all I see is the raw XML.  I'd like to be able to view the graphical execution plan the way it's shown in SQL Server Management Studio when you run a query including the actual execution plan.

I'm looking at the sample actual query plan demonstrated by Brent Ozar and want to see that style of plan on my own queries.
You need to use a product which understands that the XML represents a SQL Server execution plan.  

Make sure you're using the latest version of SQL Server Management Studio (v17.5 as of this post) and, when you click the XML for the plan returned by sp_whoisactive, you should get an extra window showing the execution plan in graphical representation.

I'd strongly suggest downloading the latest version of SentryOne Plan Explorer which is now completely free.  The latest version has SSMS add-in support such that, when you see the graphical plan in SSMS, you can right-click and choose 'View with SentryOne Plan Explorer' from the context menu and the execution plan will be displayed in SentryOne Plan Explorer. See the SentryOne post Announcing Plan Explorer Add-In Support for SSMS 2016.