Sending FX to the Mix outputs uses the exact same method as sending input channels to the Mix outputs.
Press the blue mix key for the Mix you want to send the FX to (Mix 1, Mix 2, Mix 3 etc).
In a Mix you have access to the same channels as the main LR mix.
To add FX to your Mix output simply raise the FX return fader of the mix engine you want to send.
Note: You can control the overall level of the mix engine sent to your Mix/Monitors, not control the individual send levels.