The easy way to do this in OmniFocus is to go to File > Show Backups in Finder. That opens a Finder (or in my case, Path Finder) window with your OF backup files. If you then go one folder up in the hierarchy, you’ll find Archive-ofocus.archive, which is your archive file.
Slightly faster is to hold down the option key when selecting the File menu; the Show Backups item becomes Show “OmniFocus.ofocus” in Finder, and selecting that opens the file with your active OF file and your archive.