This one is pretty easy, just like in Linux, in fact.
The problem is that Roland hasn’t released an updated (64bit) driver for the UA-25ex yet. In the meantime, there’s a switch on the back of the interface to toggle Advanced Driver mode.
If you turn this off, it makes the UA-25ex look like a standard, generic, USB audio interface. You lose volume control in OSX but at least it works.
Hopefully Edirol will release a driver soon. They’re usually pretty good about this. I know I’ll be checking their driver page for the next bit.