I'm no expert here but if you're using it as an effects processor in the effect loop of an amp, I'm pretty sure you feed the send into the input, then the output back into the return on the amp. I would make sure that you're using the presets which don't have the amp sim activated though, for the best sound.
Hope this helps!
cbroadbridge is correct, but I would like to add something to his answer.
If you want to use the XTL to add effects only to your amplifier, you would connect it as cbroadbridge wrote. From the Send of your amp to the Input on the XTL. And, from the L/Mono Output of your XTL to the Return on your amp.
If there is a level control on your Effects Loop please match it to the output of your XTL Amp/Line switch. Or experiment - if the XTL effects sound distorted you should leave the switch on "Amp".
However, if you want to use the XTL as a full amp and effect simulator (meaning you want to use the amp models on the XTL), you should run just the L/Mono output of your XTL into the Return on your amplifier. Ignore the Send jack and all your amplifier controls (except the volume!) and enjoy the wonderful sounds of the XTL.
I hope this is clear and/or helpful.