Inside the zip file is an .apk file. Copy that .apk to your sd card.
Download a file browser from the market. I use and recommend Astro, but there's a lot of choices and all are good.
Find it, and click to install it.
The market fix allows all paid and protected apps to show up in the market. It fixes the signature of the OS by changing it to the one from the N1 - one that Google knows and understands vs. some crazy Sprint testing signature.
just a fyi for Mac users, for some reason I wasn't able to install the wireless tether using a mac - have no idea why, even when I renamed it .atk. I used Parallels and downloaded to a PC desktop then transfered it and it worked great. Maybe Mac's have some issue with the .atk extension.