@ geetee1972
From one mac user to another:
Download Transmission.
Install it in the usual way.
Then go to the link Clare posted and click the little down arrow next to the episode you want.
Double click the file that just downloaded, for example Lost_S06E01_E02_HDTV_XviD_2HD.torrent. Transmission should automatically open. If it does not then you may need to point OSX to the Transmission app.
Fingers crossed the file should download eventually. If not you may have to forward some ports but that is another story.
Quicktime should play xvids fine but if for any reason it does not then head over to Perian.org and install there codecs for Quicktime.
Once the file has downloaded either leave it seeding or remove the torrent from Transmission by clicking the little stop symbol.
Somethings to be aware of, bittorrent uploads as well as downloads so you will want to limit the upload speed depending on your connection as you may throttle your connection. To do this do the following:
In Transmission go: Preferences > Bandwidth Tab (the pic of the speedo) > Tick the box next to upload rate and enter 45 to get going with.
Anything else just shout.