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. Its location is on the 3rd floor inside the Curriculum area, which is just off 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 floor maps useful. Room 387 is labeled on the 3rd floor image.