To start the quest-chain proper, look for the Argent Pavilion just to the west of the Crusaders' Coliseum. On the map provided, you will see that it is marked by "START HERE". Justicar Mariel Trueheart stands just inside and two Argent Peacekeepers stand just outside the entrance to the building. The northern part of the building serves as a chapel of the Light led by Argent Confessor Paletress. Once in awhile, someone sitting in the pews will go around a curtain to make confession with Argent Confessor Paletress.
Also you will randomly see untargetable scarabs crawl under the horde and alliance base on the grounds, hinting the fact that Anub'arak is below the grounds, waiting...