First thing would be to go to the URC website and get their archiver software for this remote, which also will update the database in the MX-450.
[Link: universalremote.com]

Once you update the remote , I would go thru the normal programming process that the MX-450 has on-screen, and maybe consider using the other wired IR ports on the back of the MRF-260 (maybe not for the TV unless it's close to the MRF). There's also consideration you need to insure the MRF-260 isn't getting interference from other components which could be causing you issues. Use the status light on the MRF and position (far away from cable box and DVD) it so the only times it is lit is when a button is pressed on the remote.