Unfortunately the 1Ds Mark III is another of those that doesn't work with tether. I've been compiling a list in this thread which you can check out:
http://discussions.apple.com/thread.jspa?threadID=1393829&tstart=75
Anyway, using Aperture to accomplish the task is pretty simple. You'll of course need to use Canon's EOS Utility software to do the remote capture. It's better than Aperture anyway, since it'll actually let you change the settings on the camera remotely as well (Aperture simply lets you click the shutter).
Once you have the Canon software set up and running, you'll choose a folder to capture the photos to. This can be any folder you like, named anything you like. Within Aperture, create a new project or use and existing project. Do the following:
1. Open Automator
2. Choose a "Custom" workflow
3. You can narrow the action list down by choosing "Photos" under library, and then find the Aperture Icon with "Import Photos" and drag it to the area on the right.
4. Choose your project from the drop down menu, then select "Import by reference" if you want Aperture to leave the photos where they are and just reference them. Leave it unchecked if you want them imported to the Aperture library. You can also choose to have them deleted after the import if you'd like.
5. Once you've set up the action with your project and import preferences, go to the file menu and choose "Save as Plug In." Name the action "Tether" or another appropriate name.
6. In the first drop down menu choose "Folder Action"
7. In the second drop down menu choose the folder where you have your Canon captured shots going and click "save".
That's it! Now every time you take a photo using the Canon software, the photos will automatically be imported to the project of your choosing. Good luck!