I was at the UA Club yesterday in concourse D before my WN flight which left from concourse C. You should take the train back towards baggage claim and concourses B/C. Then upon leaving the train, you will need to find a TSA officer to open the glass door leading to the B/C terminals. I showed them my UA gate pass and WN boarding pass and they opened the door for me. Not sure why C/ B concourses are not automatically connected to D/E, especially as the food offerings are so much better in D than C.