"What is the URL for my mobile site?"
By default, all mobile URLs follow the same scheme: http://<sitename>.prohost.mobi
<sitename> is defined by what was entered on the 'Site Settings' page in the 'Set Mobile URL' field. Note that this can be changed at any time (which will also change your mobile site's URL).
This URL is also valid if you've configured a custom domain name, and it is recommended that when integrating your site with services like Google's Webmaster Tools, that you use this default URL in the event that there are changes to your desktop site's DNS settings that might affect the integration.
However if you have correctly set up a custom domain name (e.g. m.<sitename>.com), then that custom domain name can also be used as your mobile site's URL.