The following worked for me:
Double click on the iMovieHD6.dmg file to mount it, then drag iMovieHD6.pkg to the desktop to make a copy.
Control-click on the package and choose 'show package contents'
Drag Archive.pax.gz to the desktop, then open it. You will get a folder called 'Archive'. Inside are two further folders.
From the folder named Applications, drag iMovieHD to your own Applications folder.
From the folder named Library,
Copy all 5 themes .bundles to the following path: Library>Application Support>iMovie>Themes folder
Copy the iMovie Getting Started file to: Library>Documentation>Applications>iMovie folder
Copy iMovie.mdimporter to: Library>Spotlight folder
You might find it will work without any more fiddling. Or you might not.