This is a missable achievement, which can only be obtained fairly early in the game.
After the first mission which opens the game, when you first get to the Psychonauts headquarters (the Motherlobe), you'll be tasked with heading to the internship office, but you can take time to explore. When you reach the central lobby, go through the archway labeled "Agents" then down to the end and enter the room where Milla is. Talk to her to unlock the achievement.
This achievement will no longer be available once you reach the point of attending a class and entering Agent Hollis' brain (the second major mission).