You've done nothing in that plugin to insert a navtab into your navbar. You need to use a template_hook to insert it. Something like:
PHP Code:
$template_hook['navtab_end'] .= '<li'.$tabselected.'><a class="navtab" href="link.php">Nav Link</a>'.$tablinks.'</li>' ;
You probably need to make $template_hook global also. And, you'll have to do something to the Home navtab plugin to not make it selected also since it is currently set to be the active tab is you are on a content page (you'll have to change the condition to be "on content page except these content pages....").