How to clear mac app store cache? can't download skitch

I'm Under OS X 10.8.2

Today I received an update about skitch. them I clicked the update button. the update process started.

until the progress bar reached 70% it give me an error "Skitch failed to download, Use the Purchases page to try again." them I went to the Purchases page, it give me a red text waring. "an error has occurred" I clicked the INSTALL Button again. it give me the progress bar again but for about 1-2 seconds.

the progress bar is about 80% full but reported "12.88 MB of 12.88 MB less than a minute" them the same error again "Skitch failed to download, Use the Purchases page to try again."

Now I wonder that can I do?

Clear the downloaded cache them try again?

but where is the cache?

I googled there is no Appstore folder at ~/Library/Application Support