Loading the tracks can be a bit tricky when you first start. You need to carefully read the readme files included with the zips. Once you've done one or two, it becomes second nature. There's also a lot of editing programmes available for download as well which make the process easier. You just need a little bit of patience, that's all. (You want to look out for one or two of the fictional tracks - there's one awesome one with incredibly long straights - trouble is I can't remember what it's called!!).
As far as I know, there is only a beta version of the full Le Mans track - and it is VERY much a beta version! (So beta that you can barely recognise it! I downloaded it and gave up). What you do need to remember with some of the downloaded tracks and cars is that some of them have what the experts term high poly-counts, which means that you need more processing or graphics power to cope with them than the standard cars and tracks which some with the game. My system has struggled on occasion with grids full of the downloaded cars and certain of the tracks.
By the way, I saw SCGT being sold today in Virgin for £6.99 - that is an absolute steal!!
By the way #2, I think I have said this before - if you are after a full Le Mans track, get yourself a copy of GPL and download the 1967 version of the Le Mans circuit. Granted you can't drive sportscars round it in the game (which is, I accept, a bit of a shame), but once you've driven a '76 Ferrari or Lotus down the full chicane-less Mulsanne, you will definitely have a very big smile on your face.