I can't understand for the life of me, might be better if there were a few hints for it.
Assuming you are already at the point of being able to enter Upper New Arch and are unable to find her, check your Logbook.
If it says to look for Fawn in the Slums, you need to speak with the receptionist at The Torture Rack (even if you have already done this, loading a saved game might necessitate your doing so again).
After this the Logbook will tell you to look for her in UNA, she will then appear in the college.