If you havn't already done so. Download and Install the Homebrew Channel. There is no point to explain all the steps how to do this as there are already many guides on how to do this available. A couple of good guides can be found here

Basicly in a nutshell, everything that isn't licensed by Nintendo(tm) is Homebrew, and the homebrew channel is a launch pad for all homebrew software, you only need to install the channel one time, then you can load all your Homebrew programs, from SD card and even USB Gecko using the channel!

To see a video of the homebrew channel in action. Check out this Youtube video.

Step 2. (Download and Copy Gecko OS onto your SD card)

Now you have the Homebrew Channel installed you need to download Gecko OS This version is 1.06d and can be found in the download section, if any further updates are made they will be posted there, so always check the site regular for updates.

Once you have have downloaded the zip file, extract the files to your hard drive, and you will see the files like so:

Ignore the EXE files for now, and double click on the folder which shows GeckoOS. Then double click on the folder HBC. You will then see a folder called 'geckoos' like so:

Now what you need to do is copy this folder onto your SD card under the folder 'apps'. If you dont have a folder called 'apps' on your SD card, just create one as all your homebrew apps need to go in that folder. This is very important otherwise the homebrew channel won't find GeckoOS or any other files.

Step 3. (Load Gecko OS)

With your files on SD Card, put your SD Card in the Wii's Front SD Slot of your Wii, power on your Wii, launch the Homebrew Channel and click Gecko OS. This is shown on this Youtube video.