you could use an analog to digital video converter box. One of these would let you convert the composite video output of the Wii to digital video over Firewire to your Mac and import into iMovie. I have an old Canopus AVDC (model 100? or 110, I forget) converter box which I use to import Hi-8 analog video to digital, perhaps something like that would work for your purpose?
Someone wrote a nice article on how to do it here:
They even use a Mac, so the instructions will work for you. However, one thing to note is that the Wii doesn't have HDMI, but it does have composite (and component video). I definitely also recommend the Elgato eyeTV products. They also state how well their products will work with consoles in their support pages:
EyeTV 250 Plus: http://support.elgato.com/index.php?lang...
EyeTV Hybrid USA 09: http://support.elgato.com/index.php?lang...
Comparison between the two: http://support.elgato.com/index.php?lang...