Have you downloaded Azerus yet as thaddius told you to do? Get it from here: http://www.google.com.au/url?sa=U&start=1&q=http://azureus.sourceforge.net/&e=10384
When you open it for the first time, it will probably come up with a dialog box saying if it should assosiate all torrent files with Azerus. Click yes to this. If it doesn't come up with the message, don't worry. You can still open the torrent files as long as you do it inside Azerus. However, whether the message comes up or not, try testing a torrent file by double clicking on it, and see what happens. If you did get the message it should open straight away inside Azerus, if you didn't get the message, it might open up as well. But if it didn't open up, there's a backup plan.
Open up Azerus, and you should see a series of icons along the top of the client. Click on 'Open a .torrent file' (the icon with an open folder and a lock). Now locate the torrent file and open it. If there is a valid amount of seeders and peers, your download should start in a few minutes. You can check how many seeders and peers by checking the torrent site from where you got the torrent. You will need at least 10 seeders for a large file (though you can probably get by on 5, depending on your internet connection). Peers are normally irrelevant, but generally, the more peers, the better. (Seeds means people who will share the file with you, who already have the full file. Peers means people who are trying to download the file like you, who will share what they have of the file with you, but will also download what you have of the file from you). If the torrent file is too old (you should be able to see this on the site) then beware! There's a good chance the actual file doesn't exist anymore, any files that are more than a month old you should be wary of.
Gamecube iso's are very hard to find from the net. I'll give a few basic torrent sites just to help you, but there's a limited chance you'll be able to find the specific .iso you are looking for. You're probably better off simply getting the actual game as it will save you a lot of the trouble of finding, downloading then burning the game:
http://www.torrentspy.com
http://www.torrentreactor.com
http://www.isohunt.com
Good luck, and i hope you can get your Gamecube Game! ;)