Please see this thread
for the ongoing discussion about this issue.
It's caused by a bug in Unity 3D, and at the moment there is apparently not much we can do about it. We're basically waiting on Unity 2.7 or 3.0 at this stage. Thanks for bearing with us on this, our hands are tied, and we're as frustrated by that as you probably are if you're on a Mac and don't like the default resolution of Tidalis on your machine. But, in the meantime, there is a workaround!Workaround
There is a "Use In-Game Resolution Settings" checkbox in the settings menu. To set a custom resolution on the Mac, first step is to uncheck this box, then save and close the game.
From then on, it will use whatever screen resolution is configured directly through Unity. To configure that screen resolution, hold the Option key while clicking to open the game. You'll then get a Unity screen that lets you select the resolution you want (and it seems to remember your last choice), and then you can start the game with no issues.