Q: How do i add the find my iphone app, if my phone is already stolen/lost?

You can't do anything now. Should have thought of that before it was lost.

What To Do If Your iDevice or Computer Is Lost Or Stolen

If your Mac, iPhone, iPod, iPod Touch, or iPad is lost or stolen what do you do? There are things you should do in advance - before you lose it or it's stolen - and some things to do after the fact. Here are some suggestions:

  1. Reporting a lost or stolen Apple product

  2. Find my lost iPod Touch

  3. AT&T, Sprint, and Verizon can block stolen phones/tablets

  4. What-To-Do-When-Iphone-Is-Stolen

  5. Lost or Stolen iPhone? Here’s What to do

  6. 6 Ways to Track and Recover Your Lost/Stolen iPhone

  7. Find My iPhone

It pays to be proactive by following the advice on using Find My Phone before you lose your device:

  1. Find My iPhone

  2. Setup your iDevice on MobileMe

  3. OS X Lion- About Find My Mac

  4. How To Set Up Free Find Your iPhone (Even on Unsupported Devices)

Third-party solutions for computers:

  1. VUWER 1.5.4

  2. Sneaky ******* 0.2.0

  3. Undercover 4.7

  4. LoJack for Laptops Premium Mac

  5. STEM 2.1

  6. MacPhoneHome 3.5

Actually, up until only a few days ago, this WAS possible.  My daughter had lost her phone over the summer.  I put the app on MY phone, used her apple account info and was able to track (and find) her lost phone, without adding any app or changing any settings on her phone at the time.  Was still able to track her phone up until just the other day.  NOW we're being told it has to be activated on her phone also??  Why now?  How did it work before when it wasn't on her phone and only on mine??  They need to change whatever they did recently and make it so a phone that is already lost CAN be found, without having to activate/change anything on the phone!