Adding a post shortcode to a page template?

by

To use shortcode in a PHP file (outside the post editor) you have the handy little function
do_shortcode();

so in your case you use:

<?php
do_shortcode('[mingleforum]');
?>

Update:

following the first comment,
I figured its publicly known for some reason,

if you are expecting output from the shortcode then echo it out.

<?php
echo do_shortcode('[mingleforum]');
?>