This is an issue with the combination SystemModeler 4 and Mathematica 10. It will be fixed with the next version of SystemModeler.
As a workaround, try adding this option to the WSMPlot call:
WSMPlot[\[ScriptCapitalM], {{"y", "a", 0.1}, {"z", "a", 0.1}}, PerformanceGoal -> "Speed"]
