On Mac OSX (Mavericks) you can find your ‘iCloud’ documents in the relevant application folder like this:
Each application keeps its own documents in a sandboxed folder.
You can make an alias for a file and then move that to an appropriate folder, for example, I’m using Byword on the Mac, iPad and iPhone, thus I want my ‘List.md’ file kept in iCloud. Putting an alias to it on the desktop makes it easy to edit on the Mac. Using GeekTool I can have it automagically display the contents of ‘List.md’ on the desktop, essentially a ‘live’ file no matter where I edit it, even from iDevices.