how do you hook up the wii

i saw that you can use the wii with unity but i cant hook it up

You can't just plug and play the Wii to your computer and use Unity, there are very specific things you need:

1) Unity Wii License (Mac only, based on the 2.1 feature set - similar to that of Unity iPhone)

2) Nintendo Developer Certification (Obtained through a bit of paperwork through Nintendo and an application process)

3) Nintendo Wii Dev Kit (Windows only - yes, I too see the platform discrepancy)

As a certified Nintendo developer, I cannot reveal the workflow as it is a violation of their NDA, but there are resources available to you upon certification that will clear things up.