Access methods in plugin template

I'm new to PHP and trying to learn my way through Wordpress. Within my theme's functions.php file, I wanted to access a method available inside bbPress's plugin template.php file. I'm getting "Unexpected token ['

Inside bbPress plugin template.php:

function bbp_topic_reply_count( $topic_id = 0, $integer = false ) {
    echo bbp_get_topic_reply_count( $topic_id, $integer );
}

I'm accessing this function in my current theme's functions.php:

$count = bbp_topic_reply_count(125);

Do I have to include anything from the plugin (i.e bbpress/template.php)?

Thanks.

via Chebli Mohamed