You need to make it a multi-dimensional array, with 'id' being the value that will be returned by the form, and 'text' being the displayed text in the pulldown.
Code:
 $modArray = array();
 $modArray[] = array('id' => '1', 'text' => 'Trex250');
 $modArray[] = array('id' => '2', 'text' => 'Trex450');
 $modArray[] = array('id' => '3', 'text' => 'Trex500');
 $modArray[] = array('id' => '4', 'text' => 'Trex600');
 $modArray[] = array('id' => '5', 'text' => 'Trex700');