The DCU has a 3-pin connector to connect a servo motor. The DCU line used to control the servo is line 4. The DCU has internal electronics to only allow its 4th line to be turned on when the first three lines are off. You will need to add code to your project to turn off the first three lines when using the 4th line to control your servo motor.

Please reference the servo sketch posted on the Vernier web site:

Arduino and SparkFun RedBoard Troubleshooting and FAQs