Wow.
A bit of history there. I have fond memories of mixing many live gigs on a 200B...buts it's been years since I've looked at one.
So...
4 sends and returns.
From memory, Auxes 1 & 2 are pre-fader - or at least are switchable pre/post.
3 & 4 are post-fader only.
So, assuming you've got 4 effect boxes - 'verbs & delays etc - plug the out put of Aux 1 into the FX box. Plug the output of the box into Return 1's inputs...assuming they are stereo returns.
Repeat for the other sends/returns.
For this example, let's just use aux send/return 1.
To make the effect work, you need to firstly make sure the AUX SEND MASTER for the Aux is up. IIRC, they are knobs - not linear faders. Turn it up to unity, which is about 7 on the marked scale. I can't remember if they are +4 or -10...sorry...
Turn up the send on a channel...a lot. At this point you should see levels at the input of your FX box.
Route the Return to the Main outputs - which I assume you're listening to - and set that for unity. You should be hearing the effect. You will probably be hearing way too much effect. Turn down the CHANNEL send until the effect is at a sensible level.
Rinse & repeat for the other sends/returns.
Hope this helps.
Cheers,
Tim