The easiest way to debug something like this is to look at the MediaElement module itself.
Its implement of
hook_library() contains the following:
$libraries['mediaelement'] = array(
'title' => 'Media Element',
'website' => 'http://mediaelementjs.com/',
'version' => '2.1.6',
'js' => array(
// The mediaelement script detects the path to itself to call other files
// in the same location. With preprocessing this is problematic as the
// script is no longer in the same directory as its other assets. There
// is an option that can be passed into the script with its location.
// @todo Update all calls to mediaelement to pass in the assets location.
$path . '/build/mediaelement-and-player.min.js' => array('group' => JS_LIBRARY, 'preprocess' => FALSE),
'css' => array(
$path . '/build/mediaelementplayer.min.css' => array('group' => CSS_SYSTEM),
So it would appear the module is looking for
If you make sure those files are in the right place, you should have no more issues.