Mx-450 and Apple TV won't learn menu set on remote

I've got an MX-5000 and an MX-880 and ATV works fine with them. You just paste the IR commands onto the hard buttons. And I'm using AUX 476. You should switch back to that, since your problem before had to do with pairing.

I would unpair the remote. That's only needed if you want it to ignore other remotes (e.g. you have multiple apple tvs in the same room):

http://support.apple.com/kb/HT1555

And if you told the apple tv to learn commands I would undo/reset that too.

Basically, with a new, out-of-the box Apple TV the URC remotes will work. As I said, I'm using AUX 476 and it works just fine.

So unpair all remotes and unlearn/reset commands on the apple tv. Using AUX 476 on the URC will then work.

Another thing to watch out for is the IR blasters on the front of the Apple TV. You need to make sure they are positioned properly. And in my case I have two of them in a rack, feeding two rooms. One is behind another piece of equipment and I have to cover the front of the IR blaster or else there were issues with it reflecting off the device in front. So I don't know if that's an issue for you, but check the placement of the IR blaster and cover the front if necessary. I just used electrical tape.

Good luck