The normal way to zoom in and out would be to use the thumbnail slider. I am not aware of any keyboard shortcut for this.
Here is a list of the keyboard shortcuts for iMovie.
Interestingly, in Final Cut Pro, there is a keyboard shortcut for zooming the timeline. For example, Shift-Z will scale the project to fit the timeline, and command+ and command- will zoom.
I did go into the manual before posting and read all the shortcuts. They were the same as the ones in this link you posted and I didn't find anything that says zoom. That little slider on the bottom right is a pain. I also hate the timeline following me around everywhere I move the mouse. It should be like vegas where ever you click on any clip that's where the timeline snaps to. Thanks for your reply AppleMan!