Sarah,
Thanks for contacting National Instruments.  Not all of our boards support analog triggering.  The USB-6009 only supports digital triggering.  You are also limited in your programming byusing Data Logger.  It may be possible to accomplish this task using a digital trigger in LabVIEW.
Regards,
					
				
			
			
				
	Kenn North
Principal Product Manager - Search, Digital Analytics
http://ni.com/search