I have a mackie sr32-4 and I also have a DBX - 231 EQ. I take a 1/4 cable and plug the input of the EQ to the aux send 2 of the board, and Im not sure where the output goes because the aux returns are all stereo.
Aux sends 1 and 2 are pre-fader monitor sends. They are for providing monitor signals for the performers through monitor amps and speakers. These do not change their outputs when you move the channel faders.
Aux sends 5 and 6 are after-fader send-effect sends. They are used with send effects to add new sounds derived from the sound sources (e.g. reverb). These change their outputs when you move the channel faders so the effect diminishes when you diminish the sound source.
Aux sends 3 and 4 can be switched to be either pre-fader monitor sends or after-fader effect sends. Make sure all of the channel strip switches are in the same position for what you are using the send for.