I’m reasonably certain that the rules never say. So far as I can tell, it seems you can just... use a move action to mount something. There’s nothing saying you have to be anywhere near it at all, which is dumb. (For that matter, I also don’t see any rules about the mount having to be OK with you getting on, which is also dumb. Though you do presumably still provoke from that creature for entering its square, which one way or another you have to do to wind up mounted.)
So I don’t think the rules are going to work here. To me, it seems most sense that you can only mount a creature while sharing that creature’s square. Since mounting is a move action, this works out to being almost the same thing as mounting from any adjacent square, since you can five-foot step into the mount’s square and then mount up (remember, you can five-foot step and use a move action in the same round; the restriction is that you cannot five-foot step and use a move action to move in the same round). Note also that DC 20 Ride check can allow you to mount as a free action, which means you can then mount in one turn from greater distances (using your move to reach the square and then mounting as a free action).