|You do not and to can not. The only people that can find out the individuals IP address are face book and they are not going to reveal it to anyone else without a court order.|
If you are being harassed by a face book user contact face book and get them to sort it out. They will have all the proof that is needed if what you say is the truth.
Having said that, knowing her IP address would not do you a lot of good anyone as most individuals using the Internet are using dynamic IP addresses which are likely to change at regular intervals, daily in some cases.