They land at two points on both maps. Just jump on.
Good to note - you CAN jump off, but rarely land safely. Water is always safe (if you successfully land IN it, heh) and using a mount can buffer a fall -very- minorly.

I tried jumping on and it wouldn't let me. Is there a certain position or something I need to have? Do I have to talk to someone first?

There may be a restriction on the continental highlands one if you haven't done the balloon quest at the west landing, (on border of d2/d3 on map). Other than that, pretty sure you need to enter by the front side (where there's a gap).

Might want to un-mount to hop on, since your maneuverability is better that way.

I can't find the second landing point on the Crimson Ilses map. Any hints?

There is no second landing point on the Crimson, just one.