How/Where can I add my <link href="https://plus.google.com/{+PageId}" rel="publisher" /> tag in my Website with Common Spot?
Not familiar with Common Spot but did a little investigation. You would likely want to edit your base template (/templates/template-basepage.cfm). All of the pages on the site should inherit from this file. You would insert your link element into the <HEAD> of that template so that the setting was picked up by all the pages on your site.
If you have a more complicated site with subsites that have different Google+ Pages, you would probably use the subsite's template instead and put the different link's in each subsite template.