There are a number of Dof widgets for Mac floating around but I like this one the best. It is available from VersionTracker, and it's free!

Once installed onto your Dashboard this DoF calculator is very easy to use. Flip it over and select your camera from the make and models and you're set.
Enter in the focal length of your lens, the aperture you intend to use, and the distance to your subject.
The pictorials at the bottom of the screen will adjust and give you the Minimum and Maximum distances of acceptable focus.
Take note of the Hyper-focal measurement at the top right of the window. This gives you the distance to focus at to maximize depth of field based on the focal length and aperture you supply.
Simply adjust the aperture setting to increase/decrease your Dof, and don't forget to change the setting on your camera.