The top of spring should be attached to the end of the horizontal rod which is mounted at the top of the vertical rod. The 50 g mass hanger is attached to the bass of the spring.
Try to demonstrate how omega changes with mass (omega=sqrt(k/m)). Start with a total of 100g on the spring (50g hanger+50g weight). Time 10 oscillations. Now put a total of 400g on the hanger. The time required for 10 oscillations should approximately double.
Those (few) students who take the second quarter of IPL will devote a full lab to a detailed quantitative analysis of this apparatus. As a demo, it is probably more important to focus on the qualitative result that more mass makes it oscillate slower.
If you want to devote a little more time to this, you can change to a stronger or weaker spring and show what happens.