This plugin appends "wpcf-" to the start of every custom field. Fortunately, I saw this in about 30 seconds, but I can see that others quite understandably found it a puzzle.
(1) It's a pointless behaviour that only makes things harder for people using the plugin. What possible purpose could it serve? Isn't this plugin meant to integrate seamlessly?
(2) Nowhere in the documentation or in the plugin itself do the plugin authors state that to use the custom field in a theme file, you need to append "wpcf-" to it.
If you were feeling your way around WordPress and you'd hidden WordPress's built-in custom fields module (or set up a custom post type where this was hidden), you would have absolutely no way of knowing why
get_post_meta() wasn't returning anything for a particular custom field. You'd just assume that the plugin was faulty.