To send the channels to a given output you have two options:
1) press the MIX button of the output channel and adjust the fader of the input channel to the desired level
– in this mode the faders of all input channels will reflect how much is assigned to the selected output
2) press the MIX button of the input channel and adjust the fader of the output channel to the desired level
– in this mode the faders of all output channels will reflect how much of the selected input is routed to all output channels
If you use the L C R type of Main (in Setup->Config->Mixer Config), The left, center and right outputs are always selected at the same time. The MIX LED of both outputs will be both turn on when pressing MIX on either the Main or C output channel. You can use the pan of the input channel to assign it to a output (or somewhere in between L, C or R).
If you want separate control of your center speaker, you may want to use the LR + M type of Main. In this mode, the M output can have a completely different mix.
Feel free to upload the scene (by saving to USB) here such that we could take a look.