You can re download without having to re purchase the app but first delete the app from your Mac using this free utility > Download AppCleaner for Mac - Uninstall your apps easily. MacUpdate.com
That will uninstall the app and all the associated files.
How to re download apps from the Mac App Store:
Open the App Store. From the menu bar click Store > Sign In
Click Purchases from the top of the App Store window.
Select which apps you want to re download. Then right or control click where you see Installed then click Install.
Make sure and use the same Apple ID used for the original purchase.
Or you could try it Apple's way, first, without downloading anything:
And the FAQ:
Apple's way is not always the best, but I'd exhaust easy options before downloading something unknown to solve a problem.
Also see this thread: https://discussions.apple.com/message/17632750#17632750
Your app may have been moved.
Message was edited by: Matt W (TechnicalMac)