While doing demonstrations on my MAC, either live or screencast, you want to draw attention to the part of the screen the viewer should be focusing on.
I hold down the CONTROL (seriously, control not command) button on my keyboard and then scroll on the mouse/trackpad to zoom in.
If this does not work for you, you will want to turn on the settings. Click on the apple in the upper left hand corner and go to system preferences. Once there go to accessibility. Click on the Zoom icon on the left and checkbox “Use scroll gesture with modifier keys to zoom:”