The Vex Transmitter’s Channels 5 and 6 control the Motor Ports of 7 and 8, unless a Jumper is placed on the Vex Controller.
Try The Inventor’s Guide:
Section VEX 2008 Inventor’s Guide - 7 Logic, page 7-4 (PDF Page 4)
and
Section VEX 2008 Inventor’s Guide Reference - Appendix D, Control Configurations, pages D-7 and D-8 or D-9 and D-10 (PDF pages 7-8 or 9-10).