Hi! I recently bought a used Marshal 5210 amp (transistor, combo) and now I just wonder how i should set it up to use it with my Zoom G2.1nu effects pedal. When I use it with the amp set to a clean tone the clean modes on the Zoom works good, but the distorted ones sounds really bad.

So... does anyone know how I could get it to work properly?
(havent tried the effects send/return thin on the back, because I just have 2 cables
-and are'nt they made for time-based effects?)
The Effects loop is more to modulation and time-based effects as you mentioned. Also it could be used for pre-amps and etc.

You are using the distortion of the amp or the Zoom?
Gibson Les Paul Standard

Mesa Boogie Mini Rectifier
Custom WG Cab with 2x12 Eminence Swamp Thang

Line 6 POD HD500
Sounds like you are using the Zooms amp modeling. Run your Zoom directly into your Fx return (bypassing your preamp) and see how that works for you.

Signal chain will be Guitar>Zoom>Fx Return
hi,you need four leads and try this configuration ....guitar to input on pedal,mono output on pedal to return back of amp,loop return on pedal to send back of amp,loop send to input front of amp.you may have to set the loop switch to on in your pedal,i know this works for the boss gt pedal range.good luck.
I believe the most common arrangement is:
Guitar->Tuner->Wah->Distortion->Preamp->FX Send->Distortion->"modulation" (tremolo/chorus/flanger/pitch shifter/etc.)->delay->reverb->FX Return->Power Amp->Cab
Sorry for late reply But thanks guys! Got it working by plugging it directly into the FX Return