You have a couple of options to Zoom in and out of both the timeline and the stage:
With your keyboard using
cmd +to zoom in and
cmd -to zoom out.
Using your trackpad by pinching to zoom.
cmdwhile scrolling with your mouse.
Using the top nav
View > Zoom In/Zoom Out
You can also use
cmd + 0to reset the zoom.
NOTE: To choose whether to zoom into the timeline or the stage, you need to place your mouse cursor on top of the selected area.