For an example of how to use the MultiSelectField module:

static $many_many = array ('Things'=>'ThingPage');

function getCMSFields() {
   $fields = parent::getCMSFields();
   $fields->addFieldToTab(
     "Root.Content.Related",
     new MultiSelectField(
       "Things", // Relationship that the field controls
       "Related Things", // Label for field
       "ThingPage", // Related class
       "ID", // Mapped data field in related class (normally ID)
       "Title" // Mapped label field in related class (often Title)
     )
   );
   return $fields;
}