@JamesB
The ZED-10 has XLR outputs, but also 1/4″ TRS insert points on the main output.
Insert points are not designed to be used as outputs and would require a non-standard cable to be made to get an output from them without breaking the signal path.
If you only need a mono signal, you could use the Aux or FX output to the RC300 – an added benefit to this would be being able to bring the output from the RC300 back into the mixer and control level of live vs looped audio.
You could also use the record out to the looper, which would mean you could quickly switch any channels in/out of the looper with the record switches.
Otherwise, if you just want to use the ZED-10 as a summing mixer for the input, make sure to use XLR to 1/4″ TRS (Jack) cables.
Hope this helps,
Keith.