How to use the antishake option for iMovie
Not sure if anyone is still looking for the answer to this but as far as yosemite is concerned these answers are wrong.  You simply drag the clip or movie into the timeline as normal and click either the entire clip or section that you want and then you click on enhance.  Once you do that you now make sure to click on the video camera icon and the option for Stabilize Shaky Video will show and you can select it