Make a custom taxonomy appear in adding posts (like category)

I’m developing over a premium theme , I have some already registered taxonomies. Those taxonomies appears on the right side bar when creating custom posts (or custom pages not sure) that are related to the theme (and its plugins).

The thing is, such ability to choose from those taxonomies doesn’t appear when creating ‘normal’ posts. How can I make it appear there too?

What I’ve done so far is follow this post, so I created a plugin and activated it. It have code like this

add_action('plugins_loaded','add_spTaxonomy_pt');
function add_spTaxonomy_pt(){
    register_taxonomy_for_object_type('actual_taxonomy_name', 'post');
}

?>

But that didn’t help. What should I do?