Single-clicking didn't do anything unless I held the button for a half second. Most of the time, my squad wouldn't move to the clicked location, but instead fired at that location. I was still able to complete the game, but some levels were extremely difficult.
If you have a USB mouse with 2 or 3 buttons connected to your Mac, you can run the game using the PC control mechanism instead of the Mac one if you wish. To do this, edit the file Darwinia.app/preferences.txt. You can edit this from the finder by Ctrl-clicking on the Darwinia icon and selecting "Explore Package Contents". The very last line of this file reads "ControlMouseButtons = 1". Set this to 2 or 3 and you'll get the PC control mechanism.
I believe that setting ControlMouseButtons = 3 would solve this problem, but... in the release version, the preferences file mentioned above doesn't exist. Where can I find and modify this setting?