My sister went through the same problem when she potty trained her son. He was fine naked, but the minute she would put something on him, he wouldn't use the potty. Her pediatrician said that her son (at the time was 28 months) was confused because she let him run around naked before introducing the underwear. She quit letting him go naked and made him wear underwear during the day. There were a few messes, but after a week or two he finally got the hint. :)
Thanks I guess I'll just try it again! looks like I'm going to need to buy another pack!