Sometimes the USB 2.0 port cable is not plugged in all the way on the PCB board with the actual USB 2.0 ports installed on it, like in the picture shown below.
Check and see if that cable is plugged in all the way, or just unplug it and plug it back in to see if that helps resolve your issue.
To access the I/O panel on the Define Series you will have to pull off the front panel. First make sure that all the cables that coming from the front panel have some slack.
At the bottom of your front panel there will be some space that your fingers will fit. Grab a hold there and pull then the front panel should come off, it may take a little bit of force.
If the port still do not work after having check this, please contact support.