Seeing a list of SQL Server Reporting Services reports from Report Explorer WebPart is pretty simple.  Set the “Report Manager URL” property to the URL of the report manager.  In my case http://localhost/Reports.  What I couldn’t figure out for a while was viewing a report that I clicked on from the Report Explorer WebPart in the report viewer WebPart.  This is actually simple once I figured it out.  Here are the steps.  This assumes both web parts are added to the same page.  And the Report Explorer is configured.  For help on that click HERE.

1) Click Edit “Site Actions” top right of page
2) Edit Page
3) On the Report Viewer WebPart click edit
4) Go to connections from the popup menu (follow next path in popup menu)
5) Connections -> Get Report From -> Report Explorer
6) Exit Edit Mode, top right of page

There you go you should now be able to click on a report in the Report Explorer and view it in the Report Viewer WebPart.  Hope that helps.