Matlab and SimuLink Development Forum

So I have a PCB populated with an array of LEDs hooked up toa power source and interfaced to my computer with an Elexol USBMOD3 usb module.I am trying to use Matlab to send a signal to the usb module to turn the LEDson and off. The active pin in the circuit is the “RXD  UART – Receive Data Input” pin. So far Ihave created a serial device and opened it for writing using the following:
 
LED = serial(‘COM4’)
fopen(LED)
 
I know the LEDs work because when I short the usb module theLEDs turn on. I assume I just need to send a signal switching the pin betweenhigh and low? So my questions are:
 
1.    Is this the correct pin to be using for this?
2.    What code do I need to send to the usb module toswitch the signal to high?
 
This seems simple but I am very new to this. The link to theuser’s guide for the USBMOD3 is below which will show all the possible pins Ican use. Thanks for the help!
 
http://www.elexol.com/Download/documents/USBMOD3DS1.pdf