iCloud Drive and OS X Yosemite
With the new OS X Yosemite release, Apple have introduced a new feature called iCloud Drive (just like they did with iOS 8 a month ago). This is a bit like Dropbox and promises to be a better way to interface to iCloud itself.
Unfortunately once enabled (ON ANY DEVICE!), all of your files are migrated and they become inaccessible to any device NOT running iOS 8 or Yosemite (OS X). This makes it a bit of a problem for users that need compatibility between multiple OS versions.
For now I would STRONGLY ENCOURAGE you to hold off enabling iCloud Drive, unless you can do without iCloud support in my apps.
Having said that, iCloud Drive does work, but clearly only between iOS 8 devices and Mac’s running Yosemite.
Please see Apple’s FAQ on iCloud Drive.