Open the Terminal utility (in the /Applications/Utilities/ folder) and run the following command (copy and paste it):
open $TMPDIR../C/
This will open a directory that holds various caches for applications and system services, so locate the one for the App Store, which should be a folder called "com.apple.appstore." Once found, you can either open the folder and try removing the installation package for the program itself, or simply remove the entire folder (it will be recreated when the App Store is next run).
After removing the cache or the installer within it, try running the App Store again and downloading your desired program.
