Apple's iCloud only allows iWork Documents

If the folder ~/Library/Mobile Documents does not exits

1. Open a terminal window from Applications -> Utilities -> Terminal.
2. Enter the following command:
   
mkdir ~/Library/Mobile\ Documents


If the folder ~/Library/Mobile Documents exits

1. Open a terminal window from Applications -> Utilities -> Terminal.
2. Enter the following command:
   
cd ~/Desktop

3. Enter the following command:
   
ln -s ~/Library/Mobile\ Documents iCloud_Documents


Now all files in the folder iCloud_Documents on the desktop are stored in Apple's iCloud and synchronized with your iOS devices and Macs.