The "public" checkbox makes the job viewable on your external jobs page.
This assumes that your OpenCATS system is accessible over the internet. If you have it on a shared hosting site, or a server that is set up correctly.
If your opencats installation is on a local only machine that is not configured to be accessible from other machines over the internet, then no one will be able to view your external jobs page.
As for the XML Feeds....I defer to Russ, I don't know anything about that yet