I just digged out my old Game Cube with a few controllers. Also, I found an adaptor on eBay which claimed to do this but is NOT Mac compatible.
Do you know how to connect them to my Mac?
Testing out the adaptor on a Mac might be your best shot, or you could post a question asking if the sellers can attempt to connect it to a Mac and see if it works. Generally, Game Cube controllers aren't straightforward to use on modern computers, and if the drivers aren't compatible with Macs, they simply have no way of working. I do however encourage you to try, it must be quite cool to use the old GameCube controller with your Mac.
Now, I don't know what you need to do this for, but if it's just for regular gaming, I suggest you buy a cheap USB-controller for this purpose, or use a PS3-controller via Bluetooth.