The package you downloaded is an UPDATE for iMovie. It is not the actual application itself. iMovie HD 6 used to be downloadable as a standalone app just after iLife '08 was released but is no longer accessible now that iLife '09 was launched in January 2009. The only way to put iMovie on the computer system is with the install discs it came with (assuming iMovie was bundled with it) or with a retail iLife disc.