Same here. Downloaded the viewer, pointed to the in the viewer folder index.html, and all the widgets are broken. The base map loads, but that's about it.
Basically you're running it locally from the bin-debug folder e.g. C:\Users\Windows User\Adobe Flash Builder 4\ArcGISViewerForFlex\bin-debug\index.html and your web bowser doesn't like ExternalInterface call. To solve this problem you have 2 options either Export Release Build the application and then it will run properly as it is fully built or secondly and more preferably build it on your web server and run it from there. e.g. Right click on the FlexViewer Project -> Click Properties -> Click Flex Build Path. In the Output folder textbox at bottom, specify the location of your web server where your Flex Viewer should be deployed, e.g. in case of IIS web server, C:\Inetpub\wwwroot\flexviewerdebug In Output folder URL text box, specify the URL that matches with your output folder specified in last step, e.g. http://localhost/flexviewerdebug/ For more step by step instructions for setting up ArcViewer for Flex please see http://help.arcgis.com/en/webapps/flexviewer/help/index.html#//01m30000001q000000