OK, I get it now, you type in aeroport CDG 2 TGV, not Paris. Now, I've asked for CDG to Sarlat, via Libourne, (and just picked a date in March) but it won't let me go any further, because it says it requires more than 3 stops, and to contact the station. Will I not be able to get my train tickets in advance, or what am I doing wrong?

Use "Roissy" for CDG airport when you plug in the departure point.

If it's telling you there are mroe than 3 changes, you don't want to take that route. Try plugging in CDG to Libourne to Périgueux or CDG to Bordeaux to Périgueux. There are loads more trains to Périgueux than to Sarlat and you're traveling on a Sunday, so you may not be able to do the CDG-Libourne-Sarlat thing easily or at all.