Plugins are called by hooks. If you look at the dropdown for hook locations in the plugin page, you'll see a ton of them. They are all located in the php code. So, if you pick "postbit_display_complete", you can do a search in your files and find that one of the places it is located in the file class_postbit.php around line 314:
PHP Code:
// execute hook
($hook = vBulletinHook::fetch_hook('postbit_display_complete')) ? eval($hook) : false;
So, you would be inserting code right there. You can see that several lines below that is where the postbit (or postbit_legacy) and the postbit_wrapper is rendered. So, if you want to insert something into those templates, that is a good hook location to pick.
You dont call plugins in templates, you render templates in plugins.