I have a Mackie 1202.
Just use one of the Aux sends and returns to route your signal to the reverb unit and back. If it's stereo, use L and R Aux outputs to connect to the inputs on your reverb unit. Then use the output(s) from the reverb unit to go back to the Aux returns on the Mackie. I would recommend setting the Master Aux Send level to "optimum" or straight up. Set the levels on the reverb unit until they are the right volume (not distorted). You will assign whichever channels you want to go to the reverb unit by adjusting the "aux 1 (or 2) level on the channels. Be sure those channels that you DON'T want to send have the aux level set to zero.
Getting the right levels, where the signal is clean and not distorted, takes a little tweaking, but it is not difficult.
Mackie has a really wonderful owner's guide that is free at all dealers. Or you can request one from Mackie.
DonM
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DonM