The left side of the ICBM's rear panel has two toggle switches and two outputs (labeled "R" and "L" under a heading labeled "Sub/LFE Output" - those "R" and "L" outputs are where you would connect the right and left subwoofers. The preamp connections from the receiver will of course go to the LEFT and RIGHT IN jacks, and then the LEFT and RIGHT OUT jacks will be connected to the inputs on the Mackies.

Since you will be mixing the LFE signal into the front channels at the receiver and only connecting the front channel pre-amp outputs to the ICBM's inputs, the only front panel controls that you need to worry about are the left/right crossover point and the subwoofer level. The "LFE Mix" control does not apply because you are not using the ICBM's LFE input. Since the goal here is stereo subs, you'll want to have the MODE switch set to "STEREO". The LOWPASS switch can probably be left in "NORMAL". LEFT/RIGHT RECOMBINE should be set to "OFF".