I'm assuming that you are using the latest version of CVI and NI-DAQ. Then there is a DAQmx example that ships with NI-DAQ that does a Continuous Acquisition with an External Scan Clock and Digital Start Trigger.
You can find this example in the CVI Folder: ...\CVI70\samples\DAQmx\Analog In\Measure Voltage\Cont Acq-Ext Clk-Dig Start
Kind regards,
Karsten
Applications Engineer
National Instruments