The MX-900 can't send RF commands directly to X-10 switches.
If you do send RF signals you will need a base station to convert the signal to IR that the IR543 can convert to X-10 RF signals.

Remote - basestation - IR543 - ...

Of cource you can have the the MX-900 send IR signals directly to the IR543 but the range will be short and it can't be any objects in the way.

Remote - IR543 - ...

I belive that FCC regulations are making the two RF systems inoperatable with each other, but I can be wrong