access iCloud backup app data
I don't believe there's an interface to the iCloud backup data that will let you pull it down to a computer. An alternative, if you're after a local backup in addition to iCloud, is to plug your iPhone in and use iTunes to explicitly back up the device (right-click the iPhone in the left-hand pane and select Back Up...

I have iTunes set not to sync with my phone, as I normally backup to the cloud, but I'll occasionally do an explicit iTunes backup for redundancy's sake.