iMovie would normally be kept in the Applications folder, is that where it was when you did the upgrade?
Also iMovie would have come with your computer. If needed you can reinstall it from the disk that came with the computer.
If you do not have the disk, your easiest alternative may be to purchase it from the Mac app store. To do that you will need to upgrade your operating system to Mac OS X 10.7.4 (Lion) or later, as the version of iMovie on the App store requires that.
Getting Lion from the App store is a little tricky. Unless you have previously purchased it, you need to call Apple sales and buy a download code from them, then go to the store to get it.
If your MacBook will support it, of course, you can always go up to OS X 10.8 (Mountain Lion.)
Best of luck.