Double click Assistant application in Finder window (Qt5.0.0beta1/Desktop/Qt/5.0.0-beta1/gcc_64/bin/Assistant.app). A messagebox with message "You can't open the application because it may be damaged or incomplete." and Assistant does not start. In 2012-08-03 version the Assistant binary itself seems to missing from Assistant.app/Contents/MacOS directory.
The same error happens with the MaintenanceTool.app, but in there the binary exists under MaintenanceTool.app/Contents/MacOS/ and can be launched from command line but not from Finder.