The communication channels which support the internet traverse great distances, and errors and outages are common. If too many errors occured while you were downloading one of our apps, Mac OS X may report that it cannot launch the app because it is damaged or incomplete. It may also happen spontaneously on a hard drive, although in that case, you should take it as a friendly reminder to make sure that your data backup system is working because your hard drive may be about to die.
In either case, the solution is to trash the application package you have, re-download and reinstall.
• Find the application package. Usually it is in your /Applications Folder, and you can get there by activating Finder and clicking in the menu: Go ▸ Applications. Leave the window open because we'll need it later.
• Drag it to the Trash.
• In the right sidebar of this page, click the Zip Archive of the app you want.
• Wait for the zip archive to finish downloading.
• Still in Finder, click in the menu: File ▸ New Finder Window.
• Navigate to where your downloads normally go, which is usually your Home's Downloads folder. To get there, click in the menu: Go ▸ Downloads.
• You'll probably see the application package, recognizable by its icon. If you don't, find the BookMacster.zip file and double-click it. The application package should appear.
• Drag that application package to your the original window where the trashed application was.
• Wait for the progress bar to complete before attempting to launch BookMacster.
Most of the above instructions are shown in a short screencast.