Used the free generator last night, followed all steps & confirmed Sitemap url. It is visible to me...no problem. GWT showed a good status almost immediately but of course didn't actually index the urls yet. I checked my GWT account today & found this:
URL timeout: DNS lookup timeout
We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.
I resubmitted & again showed a "green"/good status. We'll see what happens this time.