How do I make a Mask? (RESOLVED)

I'm new to Illustrator but I've asked a friend who works with it everyday for years and years and even she couldn't figure out how to do it. So have to ask here.

I'm simply trying to make a part of my object transparent by using a mask. This seems like it should be doable? The pink object is the one I want to be transparent on the top section. I thought I only had to make a transparent object ontop, select both and then hit Make mask. but...

I do have a video of what I mean here.

http://www.graffiticreator.net/orders/mask.mp4

Please help me understand.