[john.gersh]

As far as I can tell diagram styles are simply templates with Auto Layout turned on. If you create a new template, turn on Auto Layout and save it to the default location, it becomes a Diagram Style.

The default template folder is now
~/Library/Containers/com.omnigroup.OmniGraffle6/Data/Library/Application Support/The Omni Group/OmniGraffle/Templates

You'd rarely have to go there in the Finder though. New templates are saved there by default. The action (gearwheel) pulldown in the Resource Browser lets you do stuff to the files there (e.g., deleting them). I suppose you might have to open the folder to put in a template file you didn't create yourself, but you can open it from the Resource Browser. (Select a template and click Show In Finder from the action pulldown.) Even better, if you open a template file from somewhere else, OG thoughtfully offers to move it to the default template folder.

There's a subfolder there called Automatic Layout, which contains the default 7 diagram style templates. You can see it in the Resource Browser. It appears, though, that the Choose Diagram Style window shows templates anywhere in the Templates folder that have Auto Layout set.