If you have a WS model Mits, you can learn the discretes from the Mits remote. Do the following to set up your Mits remote:

Set the Slide Switch at the top of the Mitsubishi Remote to TV.
Press and hold the POWER button, and while holding the power button enter 090.

Now the remote functions like this:

Power - Power On Only
Stop - Power Off Only
Pause - AV Reset

Pressing INPUT followed by:
PIP - DTV
1 - Input 1
2 - Input 2
3 - Input 3
4 - Input 4
5 - DVD / Component 1
6 - Component 2
SQV - Input STB
QV - Input VGA
PIP CH Up - Antenna A
PIP CH Down - Antenna B

Learn the input commands to your MX-500 remote.
When done, press and hold POWER button and enter code 000 on your Mits remote to return to its normal functioning mode.