Can't get shortcode to display in PHP

I’m attempting to hard code a download category into a template file I’m working on, and it just displays as a block of text “{wpdm_category=test-category}”. I can only get the shortcode to appear if I put it in the WYSIWYG box in Admin.

Am I missing something, or doing something wrong? How do I make the shortcode work with PHP? I’m currently using the following:

<?php echo do_shortcode('{wpdm_category=test-category}'); ?>

Thanks

http://wordpress.org/plugins/download-manager/