Just replied to a similar question here:viewtopic.php?f=56&t=53495
The problem is probably that the MS-20's output is line level (max 3.4V peak to peak) and the System-1m's EXT IN is wired for a 10V peak to peak signal.
A core issue is that there is no output from the MS-20's oscillator(s) directly, before the final output stage, as it (most likely) operates on 10V peak to peak internally. At least most of the CV signals are 0-5V or -5/+5V.
So your best bet is to find a way to amplify your signal up to 10V peak to peak.
This is easy in the Eurorack world (which the System-1m was designed to interface with), but not sure what your options are if you don't run eurorack. Maybe there are some guitar pedals that can be used?
PS: Once you get to the point where you want to run the MS-20's pitch CV out into the System-1m, you also have the issue of two different ways of interpreting pitch CV. The MS-20 uses Hz/V whereas the System-1m (and mostly the rest of eurorack) uses V/octave. The company Harvestman made a eurorack module called English Tear to help with this: http://www.analoguehaven.com/theharvestman/englishtear/