In Mac OSX Lion, I opened Launchboard and deleted an app that I had previously downloaded from the Mac App Store
I was no longer able to download it, always getting the same annoying message from the Mac App Store software:
“A newer version of this app is already installed on this computer”
After spending hours trying to find the fix that does the trick (and, man, did I try them all: removed spotlight indexing from my HDD, removed launchpad db files, removed various folders and files), countless Trash emptied and reboots, I finally found the fix.
Here’s what worked for me (via stackexchange):
I opened up Terminal.app, pasted the line below, then pressed Enter:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
A few minutes of waiting, and when it was done I launched the Mac App Store app and.. miracle: I was able to, once again, download my favorite app from the store.
I hope this will save a few hours for you as well