Hi,
Unfortunately there is no specific function in iMovie 10 that would enable you to create a true time lapse effect from a video clip. You can use the speed tool to increase the speed of the video by 20x or more, but that would create more of a fast forward effect than a true time lapse look. (Maybe that's sufficient for your purposes.) To create a true time lapse effect, however, you would need to export your video into frame sequences, and have software that could select and stitch togeter, say, every 10th frame that you could then export to a video. You could do frame sequences in iM6, but not in later versions. Quicktime 7 Pro can do it. You can buy that at the Apple On-line store.
You might find this instructional video to be helpful:
https://vimeo.com/86859692
Good luck with this. Post back and let us know if you were able to convert your vid to a time lapse. Others might be interested.
Best,
Rich