Once the download complete, double click on the image iMovieHD6.dmg to run the set up. You will be asked for your password.
This version will be installed under Applications/iMovie(previous Version) and the 2008 version will not be deleted.
Using iMovie HD 6 or iMovie'08 after import
As both versions are installed on your PC, after your import with 06 versions, you can opt to use the 08 version by import your video from iMovie 06. To do so, go to menu File>Import iMovie HD project.