Kjuly's answer is 90% correct (and +1 to Kjuly!).
If you look at the at the Appirater code, and in the .m file that has the function that opens up the app store (it's the "rateApp:" method) I see this bit of code:
+ (void)rateApp {
#if TARGET_IPHONE_SIMULATOR
NSLog(@"APPIRATER NOTE: iTunes App Store is not supported on the iOS simulator. Unable to open App Store page.");
#else
So no matter what you try, you are not going to be able to open the App Store app within the simulator. The App Store app does not exist in the simulator, so that's why you get a "not found" error when your web view attempts to redirect to it. This should work fine on the actual device that has the App Store app installed.