FlashHax is an exploit for the Wii that is triggered by using the Internet Channel. Unlike other exploits, this one doesn't require an SD card to trigger the exploit.
1. Open up the Internet Channel on your Wii. If you don't have it, download it from the Wii Shop Channel.
1. Go to flashhax.com in the browser and bookmark the page.
1. Open the site from the bookmark. It'll then proceed to download and load the HackMii Installer. It may take some time as it is downloading the installer over the internet.