How to create a dolly zoom effect with iMovie 11

1. Get Advanced

In order to work with a blue or green screen clip you need to turn on the Advanced Tools. Go to iMovie > Preferences (or use the Command + , keyboard shortcut).

Make sure the General tab is selected (first on the left) and tick the Show Advanced Tools box to give you more editing and organising options.
2. Stack up your clips

Choose your backdrop from an event or from your photo library, and add it to your project. Next, find your subject and drag it over the backdrop. You'll be given a series of options. Select either Green Screen or Blue Screen – depending, of course, on the colour of your backdrop.
3. Apply Ken Burns

Select the backdrop clip (to give it a yellow highlight around it), then click on the Crop tool in the tool bar (or simply hit the c key on your keyboard). This tool offers you three choices at the top-left of the main preview window: Fit, Crop and Ken Burns. Make sure the latter is selected.
4. Zoom Out

Now you should see two rectangles, one green and one red. Make sure the green one (your starting frame) is smaller and inside the red one (the clip's last frame). This will give viewers the illusion of the background zooming out over time. Now click the play button to preview it.
5. Fine-tune the Look

Click Done to set the effect. Be sure to check what it looks like with your subject over it. The effect may work best if either the top or bottom of the frame stays the same (right where the green and red rectangles meet) so go back and tweak your zoom until you're happy with the result.
6. Zoom In

You may also find that zooming in on your subject over time can improve the effect. You can apply a Ken Burns effect to your chroma keyed clip too, only make the change more subtle and make sure the end frame is smaller than the start frame. Your dolly zoom effect should then be complete.