Hi Clay,
I was able to successfully submit a sitemap to Yahoo. Please note if submitting from localhost, yahoo will return the error: invalid web URL. In the Submit Site Map window within Dreamweaver, update the XML site map field to your IP address and try submitting again. If this still is not working, please let me know what errors are being returned, along with the web address in the browser that was opened when trying to submit.
Thanks,
Todd