Add function to add element on all page

To do this you have to create a function in your plugin. To have it display only to logged in users use:

   if (is_user_logged_in()) {

then use the wp_head and wp_footer links to add actions to display the link you want, ie:

 add_action('wp_head','your_function');
 add_action('wp_footer', 'your_function');