I have Imovie 09, not sure if it works the same way in 08. There are two ways to access clip adjustments. Click on the small blue icon in the bottom left corner of the clip, or click on window at the top of the screen, then clip adjustments. That will open the clip inspector where you can adjust the speed.
Hope that helps.
Double cllcking the clip that you want to adjust is yet another way to access the clip inspector, but the other two methods give you more options to other adjusting tools such as cropping, rotating, trimming, ect.