Digital Signal Generation - Clock Connections

I'm trying to output a square wave using an NI PCI-6534 card, and have this loop indefinitely until the user interrupts the process. I have a LabView code which transfers the signal to the card's buffer and outputs using the card's internal clock, and I would like Matlab to operate in the same way. I've created the session, added the digital channels, added the signal to the queue and tried starting the background process. However, I'm told that I need to add a clock connection to do this. Unfortunately after looking through the documentation, I can't figure out what the proper terminals are that I need to use as the source and destination for the clock connection.

I'm trying to use the card's onboard clock, so I tried using 'Dev1/MasterTimebase' as the source, and 'external' as the destination. This produced the following error when I tried to start the background process: "To use the session object in clocked mode with the digital output channel 'port0/line0', an external clock must be supplied."

I also tried switching the source and destination, which produced the following error when I tried to start the background process: "Hardware does not support the specified connection. Check the device's user manual for valid device routes and pin-outs."

Does anyone know the proper terminals I need to be using, or where I can find detailed documentation on this?

For reference, I am using Matlab 2013b with NI's DAQmx 9.9 drivers.