I used the link posted by Pilgram and it worked but not
exactly as described. Try

1) Press and hold Power
2) Enter 0,0,0,9,0
3) Release Power

If this does not work. would you mind posting the exact
set of keys you are pressing to determine if the Mitsu
remote will send the descrete codes including such items
as if the device/input button has a top and bottom and
which one you are pressing.

Also, have you used the learn function on the MX-500 before?
How about learn on the Cinema 7? I taught the commands
to and old Cinema 6 and then downloaded them. We might
be able to give you commands to enter on Cinema 7 that
could then be used to teach MX500.