I upgraded my macbook and I lost imovie. How can I reinstall it?

Howdy anaguajara,

If you are having trouble finding the iMovie application on your MacBook Pro, you may want to use Spotlight to search for it. See this article for how to use Spotlight - Mac Basics: Spotlight helps you find what you're looking for.

For some reason iMovie may be gone from your computer. If you have previously downloaded it or upgraded it from the Mac App Store, you can download it again at no charge. See this article - Download your past purchases.

Specifically -

Redownload an app

On a Mac or PC

You can redownload apps for iOS devices in iTunes and apps for you computer in the Mac App Store.

From iTunes:

1. Open iTunes.
2. Click More in the upper-left corner.
3. Choose Apps from the menu.
4. If prompted, sign in with your Apple ID and password.
5. Under Quick Links on the right side of the App Store, click Purchased.
   
6. Select Not in My Library to view purchased content that isn't on your computer.
   
7. Scroll to find the item that you want to download.
8. Click the Download icon in the upper-right corner of the item you want to download. Your app downloads to your library. If an app is already in your library, the Download icon won't appear.

From the Mac App Store:

1. Open the Mac App Store.
2. From the top of the window, click Purchases. Your purchased apps will appear.
   
3. Scroll to find the app you want to download.
4. Click Install or Download to the right of the item you want to download. Your app downloads to your computer.

Thanks for using Apple Support Communities.

Best to you.

If iMove shows up in the Purchased list in iTunes (meaning you have rights to use) but won't install cause of the OS, upgrade to Yosemite 10.10 or El Capitan 10.11.

If my guess is right, the OP/Apple installed it via the "other disk" that comes with Leopard (and Snow Leopard) back in 2009, not an iTunes/AppStore purchase.

The value of keeping a good clone to get back what you had and upgrade to your own schedule ...