The Performer mixer has tiny items on the top of each channel where one can select output (for the master channel) and inputs (for the input channels, with the SE version, that would be “Mic” and “Instr”). If Performer uses for instance “Stereo In 1+2” but has his/her mic connected to only Input 1, you’d hear it all to the left. Set the Performer Channels’ input to the desired hardware mono input and all should be fine.
If Performer sees moving meters on his/her “MASTR” channel but hears nothing, that is most probably a similar problem: his/her monitor device/port is not assigned properly. Again, on top of the “MASTR” channel, select the hardware output where the Performer should hear things.
Hope that helps.