Found a fix!!!
Closed App Store, Deleted Library/Caches & user/Library/Caches/com.apple.appstore, then restarted App Store. Downloaded just fine after!
Found it from ceejayoz's comment here: [URL="http://www.tttp://www.tuaw.com/2011/01/06/mac-app-store-reporting-error-100-heres-what-to-do/[/URL]
Found a fix!!!
Closed App Store, Deleted Library/Caches & user/Library/Caches/com.apple.appstore, then restarted App Store. Downloaded just fine after!