This should be ideally posted in "Ant" forum as the question is more specific to Ant, instead of webservices. I'm not answering why your taskdef failed, but below is what I'm using (taken from Ivan's notes) for wsimport and it works like a charm. May be this would be helpful for you to get going, until your real issue is fixed.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com