In order display a legend to your MTEX plots you need to add the option DisplayName followed by the text that you would like to be displayed in the legend. Objects that are plotted without this option are not displayed in the legend. Eventually the legend is displayd by the command legend
% this is a point
plot(vector3d.X,'upper','DisplayName','a point')
hold on
% this is a fibre
circle(vector3d.X,'DisplayName','a fibre','lineColor','r','linewidth',2)
% this will not show up in the legend
plot(vector3d.Z,'Marker','p','MarkerSize',30)
hold off
% display legend
legend
The legend command suport a wide varity of porperties to adjust the position and appearance of the legend.