If you have recently had your hard drive erased/replaced, or accidentally deleted an Apple application, they can be retrieved fairly easily but you will need to know the Apple ID it was originally purchased or redeemed under and the password.
Note: This tutorial applies to Mac App Store apps, such as Logic, Final Cut Pro, iMovie, etc.
Step 1. Open the Mac App Store.
Step 2. You will need to be logged in with the Apple ID that you originally used for redemption. If you are not signed in, make sure you are on the "Featured" tab (located along the top) then click the Sign In button located along the right side under "Quick Links." Note: If you do not remember your ID, go here: https://iforgot.apple.com/appleid
Step 3. Once logged in, click the "Purchased" tab along the top.
Step 4. click on the "Install" button next to the app you wish to re-install, and wait for the installation to complete.