DS is confused by training pants (he's 18mo). We use diapers when we go out, and he doesn't understand the difference between trainers and diapers, and why it's okay to pee in one but not the other. Since he was 15mo, we have him naked whenever we're home (and he's awake) and he uses the little potty. At first we would physically put him on it every half hour or so, now we verbally prompt him and now and then he'll go with no prompting. He'll also go inside whatever receptacle is available to him at the time - dog's water bowl, toy basket, a cup...and just this morning while I was showering he took the toilet brush out of the holder, peed in the holder, and dumped the pee in the shower with me. Heh! What're you gonna do :)
Anyway, I'll throw in with everyone suggesting naked time. I'd save the training pants and undies for when she's got a good handle on the potty thing, goes without prompting regularly, and can understand that pee goes in the potty *all* the time. Expect it to take some time...DS took to the little potty so well at 15mo, I thought for sure we'd be out of diapers in a few weeks. Ha! It seems like lately it has really clicked with him, he's holding it for 1-2 hours at a time, we only miss a couple times a day, but I don't think he'll be completely out of diapers for another several months yet.
We bought the Gerber training pants (look like undies, not the waterproof ones) in the smallest size (2T) and Carters underwear (I *hate* licensed characters on underwear (and kid things in general) and they were the only ones I could find) and they fit him fine, and he's a smack average 18mo.