It appears that the Newport LTA-HS actuator utilizes a simple DC stepper motor. This means that you will need both a step and direction output from the USB-6341. You should be able to do this by sending pulse trains, but how you send those trains is going to depend on what your motor is expecting to receive (which you may have to do a bit of research on).
If you search digital pulse train on the community portion of our website, there should be numerous examples of how to code them up in LabVIEW. Even so, here is an example that would be pertinent to what you are trying to do:
Writing a set of pulse trains to control your actuator will most likely not be easy simply because it will be dependent on what sort of commands the actuator is expecting. In order to determine those, you can either get in contact Newport or try to find it in the user manuals for the device.