If you want the red and green lines to show, you need to plot them separately, and create handles for them to pass to the legend call:
hold on
kr=[0 10 20 30 40 50 60 70 80 90];
M1=[0 0.2757 0.4979 0.4989 0.4481 0.3644 0.2910 0.2228 0.1380 0.0346];
M2=[0 0.0241 0.0916 0.1786 0.2613 0.3322 0.3895 0.4343 0.4658 0.4809];
plot(kr,M1,'x')
hp1 = plot(0:90, interp1(kr,M1,[0:90],'spline'),'r','linewidth',1.5);
plot(kr,M2,'x')
hp2 = plot(0:90, interp1(kr,M2,[0:90],'spline'),'g','linewidth',1.5);
grid on
xlabel('heel angel')
ylabel('arm')
legend([hp1 hp2], 'ld','ls','Location','northwest')
title('Diagram stability')
.
