Strangely, I just ran into precisely the same problem, just trying to set up my Yamaha UX16. I have OSX 10.5.8 as well. When I try to install the Yamaha USB Driver (I tried both the most recent version um121mx and the one prior um120mx), it tells me I have been successful! But then when I look where the manual tells me to, to find the installed driver (Library-->Audio-->MIDI Drivers), the folder is empty.
Actually, I used to have OSX 10.5.6, but then my old hard drive crashed and the Apple Store gave me a little update. And the UX16 worked fine on the old drive.
It's really annoying. If anybody has any ideas, I too could use some guidance.