OK, I have this figured out. Evil Dave is right that DOM3_CONF is the key. But there's a bit more than that for Mac users.
if you'll recall, Mac applications are actually bundles (a directory with various support files and the binary somewhere inside it). So setting the environment in your shell isn't going to help you when you double-click from the Finder, which is the common way to launch files.
I stand by my original claim that the game should be putting things in ~/Library/Application Support/dominion3 by default. But as a workaround, Mac users can do this:
(1) Before running the game for the first time, start Terminal.
(2) Type: mkdir ~/.MacOSX
(3) Create a file called "environment.plist" that contains the keys you need. The safest way to do this is with the Property List Editor which ships with the developer tools. But since most users don't have that, you can copy and paste the text below into your favorite text editor, and then save it as environment.plist in the ~/.MacOSX directory:
Code:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>DOM3_CONF</key>
<string>/Users/peterb/Library/Application Support/dominions3</string>
<key>DOM3_SAVE</key>
<string>/Users/peterb/Library/Application Support/dominions3/save</string>
</dict>
</plist>
(obviously, substitute your own username for "peterb". It's probably OK to leave off DOM3_SAVE -- I think it will just stick things in the DOM3_CONF/savegame directory, then. But I like the name 'save'.)
(4) Log out and log back in. Now when you double click, the app will find the directories in the right place.