I've been on both sides of this problem. I don't use a treadmill, but at my last apartment, any time I worked out, my downstairs neighbor would pound on the ceiling. I tried to talk to her about times when my workouts wouldn't bother her (and I tried extra mats, etc), but she wasn't interested in working with me AT ALL. Happily, I moved and now live in a ground-floor unit, so I can pound away on the floor to my heart's content.
Unfortunately, my upstairs neighbor has a treadmill.
Unless you've been in an apartment with someone directly above you on a treadmill, you cannot IMAGINE how disruptive it is. The whole room shakes, plaster dust shakes out from the light fixtures, the fan has started to break loose from its screws! The walls vibrate. We've had books shake free from their shelves. The dogs go berserk. Since their treadmill is right over our bedroom, we can't sleep while our neighbor is running. It sounds like we're being bombed, and he's not a big guy or anything - it's just LOUD and the whole room vibrates with every footfall. I guess I should add that we don't live in a tenement - it's a decent building and is well-maintained, so it's not like the noise is related to shoddy architecture or something.
We asked him if he could please stop running after our usual weekday bedtime, and he agreed. Sometimes, it still disrupts naps or work, if one of us is working at home (our office desk is in the bedroom), and it's definitely noticeable from other rooms, but since he was nice about changing the timing of his workouts, we tolerate it. But seriously, now that I've experienced it, I have sympathy for my former downstairs neighbor (I thought she was completely nuts). "Tolerance" is the best you can hope for.
My suggestion is to be as kind and accommodating as possible, since many apartment complexes and condo associations have actually forbidden treadmills because of the number of complaints about them.