It is very easy to program the key on a 2000 Range Rover. I am assuming that you have checked the key number on the back of the key. You can only have one of the same number (ie, 1 key that is #1, 1 key that is #2, etc). The keys are numbered to correspond with the seat and mirror memory positions. All you need to do is insert the key in the drivers front door. Depress and hold the lock button on the remote and turn the key in the direction (counterclockwise I think) to lock the door. Hold the key in the rotated position for a count of 2 seconds. Then depress and hold the unlock button and rotate the key to the unlock position in the door lock. Hold in that position for aproximately 2 seconds. This should program the remote. If not there may be underlying issues (duplicate remote numbers, inoperable remote, BeCM communication problems, etc). Hope this helps.
By the way, this should be outlined in the owners manual. The remotes often forget how to work and have to be "re-synchronized". That is all you are doing to get your new remote to work. Hope this helps.
Stephen