CONTAINS SPOILERS

To get to Andariel's Lair, you must first enter the Jail in the Monastery Barracks. Go to the end of the Jail and enter The Inner Cloister (Waypoint here) which leads to the Monastery Catacombs. There is another Waypoint that can be set in Catacombs Level 2 so look out for it. Once you reach Level 4 of the Catacombs, Andariel will appear near the entrance. Defeat her to make it to Act II.