Where is the Gare du Nord (Noord) Bus Station in Brussels?

Just below the Gare du Nord/Noord Station in Brussels (on Rue du Progrès), there is a multi-bus stop. 

And by below, I don't mean south on a map, but actually underground! 

You can purchase tickets for the Zaventem Airport (BRU) at the De Lijn office inside (credit card OK), and the bus stop itself is on the other end of the building, about a minute's walk away. 

Follow the sidewalk and look for the bus stop containing 471. There should also be a marquee showing upcoming buses and their times.

Down in the depths of the station, ye shall find it.