Ah, yes... the elusive room 387, more difficult to find than Harry Potter's Room of Requirement. Room 387 is a classroom in the library that is utilized by many departments on campus. It's location is on the third floor inside the Curriculum area, which is just to the right of the stairs. Once inside the Curriculum area, proceed to the back left corner (you will pass the information desk and the book shelves), turn down a narrow corridor, and wish really hard for a classroom with desks and a whiteboard.
You may also find the library floorplans useful. Room 387 is labeled on the 3rd floor image.