Please read the note in Step 8 first.

Procedure: Also here: http://www.xoutpost.com/articles/x5/e...ob-bmw-x5.html or here: Program and Reprogram Your Car Remote Key Fob Control Transponder - Program Your Keys Free Instructions - BMW X5 Key Remote Control Fob Programming
1. Charge the key for about 1 hour in the ignition.
2. Enter the vehicle and close the doors
3. Briefly turn the ignition on (key to position 2) and off (5 times within 5 seconds). Don't actually start the car.
4. Remove the key; press and hold the 'unlock' button. While holding the 'unlock' button, rapidly press the 'lock' button 3 times within a period of 10 seconds.
5. Release the 'unlock' button.
6. The central locking system activates and releases all locks in rapid succession to indicate that the initialization has been successful.
7. To initialize additional remote control keys (make sure that no more then 30 second elapse between each initialization procedure) repeat steps 4 and 5 ONLY. Do not repeat step 3.
8. NOTE: You MUST initialize all the keys (at least that is what I found, otherwise by initializing one you de-initialize the others).