Ok, first off, this setup isn't really meant to add reverb while you're recording. It's more meant to add reverb to tracks while you're playing back. It's not that you can't add it while you're recording (which is "printing" the effect to tape), but the benefit of the 388's FX loop is that it allows you to constantly adjust each track's amount of reverb after the fact.
So ... I don't know how many tracks you have, but here's an example. Let's say you have Guitar (TRK 1), Bass (TRK 2), Keys (TRK 3), and Vocals (TRK 4), and they've all already been recorded.
Run a cable from the EFFECT OUT jack (1/4" jack) on the 388 to the left or mono input on your reverb pedal. (There's no need to use both inputs on the reverb pedal, so no need for a Y cable.)
Run cables from both outputs of the reverb pedal to EFFECT RTN 1 and 2 on the 388. (so use two cables: out 1 to return 1, and out 2 to return 2)
On the reverb pedal, set the controls so that it will produce plenty of reverb.
On the 388, there are four blue EFFECT RTN knobs to the left of the 8 REC FUNCTION lights. Turn the two GAIN knobs to around 12:00 to start, and pan 1 and 2 hard left and hard right, respectively.
Turn the blue EFF knob in the MASTER section (right below the REC FUNCTION lights) to about 12:00 to start.
Press the STEREO button just above the MONITOR/PHONES knob.
Plug headphones in and set the MONITOR/PHONES knob to a moderate level (3 or 4) to start.
Push up the track faders for tracks 1-4 to around 7, push the L R ASSIGN button, and move the switch at the top of the channel strip to RMX
Start playing back the track, and make sure you can hear signal from each track.
Now, as you turn up the blue EFF knob for each track (just above the ASSIGN switches), you should start hearing reverb applied to each instrument.
Please try that and let me know how it goes.