bbPress.org

Well, that function gives the URI of the stylesheet, which will be a file, not a directory (sorry). Try this instead:

<link rel="stylesheet" href="<?php bb_active_theme_uri(); ?>css/enplus.css" type="text/css" />

You had some other errors in there too, in addition to the wrong function I gave you.

There’s lots of neat stuff in bb-includes/template-functions.php – take a look.