You don't use $mymenu anymore - you need to use the new syntax. The new syntax is {vb:raw mymenu} . Check the functions.php page where the navbar is rendered and make sure your hook is located *before* that.
Thanks,
You would think the hook should be navbits_complete, if I am reading this correctly.
/**
* Renders the navbar template with the specified navbits
*
* @param array Array of navbit information
*
* @return string Navbar HTML
*/
function render_navbar_template($navbits)
{
global $vbulletin;
$templater = vB_Template::create('navbar');
// Resolve the root segment
$templater->register('bbmenu', $vbulletin->options['bbmenu']);