//set up load.php inside /wp-content/mu-plugins, then add:
require(WPMU_PLUGIN_DIR . '/myplugin/myplugin.php');

//OR –– use below for multiple mu-plugins inside a subdirectory
//
// Opens the must-use plugins directory
$wpmu_plugin_dir = opendir(WPMU_PLUGIN_DIR);

// Lists all the entries in this directory
while (false !== ($entry = readdir($wpmu_plugin_dir))) {
$path = WPMU_PLUGIN_DIR . '/' . $entry;

// Is the current entry a subdirectory?
if ($entry != '.' && $entry != '..' && is_dir($path)) {
// Includes the corresponding plugin
require($path . '/' . $entry . '.php');
}
}

// Closes the directory
closedir($wpmu_plugin_dir);