Here are the steps on "lighting up" a selected navigation link in Community Server after we create and determine the url to our site article. That's how the question was asked and I like the term "lighting up" so I'm using it here. We want to make the menubar link to a content page be the selected tab in other words. For our example the site article url is getablog.aspx and we've created the link in SiteUrls.config that looks like this.
<link name="getablog" navigateUrl="/content/Getablog.aspx" text="Get a blog here!" />
1) We start with \themes\[theme]\content\content.aspx. Notice the MasterPageFile item. We're going to make a copy of that master.Master file, add a SelectedNavigation link to the copied file, and update the MasterPageFile designation in this content.aspx file.
2) Make a copy of master.Master, name it anything you want. We're naming it getablog.Master for consistency.
3) In getablog.Master we add a SelectedNavigation entry in the Header region. Notice the Selected="getablog." That's the name of our siteUrls.config link which will be selected when that master page is used.
4) We return to content.aspx and update the MasterPageFile designation.
Our new Menubar link is now selected when viewing the content page.