Right, what you need to do is this
GpXpatch doesn't actually install the tracks for you, it just allows you to use them once you have installed them. Its a very manual proceedure though
1. Unzip all the bits and bobs, in there there should be .txt file usually called gp3form.txt and this will tell you what to do.
2. Say the readme file asks you to make a new directory in your GP3jamsh folder (which is inside the grand prix 3 directory) then put all the .jam files into that, then do that, as this is where gp3 will look for the textures for the downloaded track. You will be asked to give the folder a specific name, DO THAT! the track will not work if the name of the folder you just made is not identical to that described in the readme file
Important, if you use Software rendering mode, then you need to download software optimised version of the tracks, and the folder with the jams in will have to be placed in the gp3jams folder (note the h is missing from this folder)
3. Place the trackfile in the circuits directory, in your grand prix 3 directory, it is not necessary to rename it to the slot of the track that you want to replace.
4. Go in to GpxPatch and in the GpxTrack tab and click on edit, where it says tracks set and the filename in the box should be tracks.set, this is how GpxPatch stores the data, which is inserted into gp3 to allow you to use the new tracks. After clicking edit you should have a diologue box which, you can select a new track for any slot. Say you want to replace Imola, which is in Slot 4 (F1ct04) just click on the open button next to it, find your Grand Prix 3 directory and the circuits directory, where you placed the new track file, just select it and it is now loaded in to GpxPatch, Just make sure the 'catch, menu strings' box is checked in GpxTrack and you should be able to click 'Start Gpx' and run your new circuit
Hope that helps you out, if you have any more specific problems, post again
Nik
This week I av been mostly listening to : Jeff Buckley - Grace