Taran,
Head over here for step by step instructions...
http://planeplotter.pbwiki.com/
Also am I right in thinking you dont have a ASDB receiver (SBS-1 or Radarbox)? In that case the trial version of PlanePlotter wont help you. You need the full registered version to set up the application to receive the tracking via the internet. Don't worry that you have to outlay $'s for the application. There are now a strong and active group of us using PlanePlotter in Australia and we are a very friendly gang. You will soon find it is the most insanely great software you have ever purchased.
Cheers, Mark