Unfortunately, Macintosh does not (yet) support complete iCloud synchronization for documents. We can only hope that Apple will implement this at some point soon.
In the mean time, go to http://www.icloud.com/ and sign in. You can drag documents to and from the appropriate section on your computer, and it'll work with your iPad. There's still another bug, which involves Mac > Mac syncing, but as long as you just do iPad > Mac or Mac > iPad, it'll work fine. :)
Also, there is a folder located at ~/Library/Mobile Documents/
which contains your iCloud-enabled apps' data. This includes the iWork suite, but unfortunately iWork documents aren't saved as the normal filetypes (e.g. ".pages"), so they can't be opened on their own.
Doesn't look like there's a way to single out the Desktop via the 'Manage Storage' GUI.
However, you can always just symlink any directory from your iCloud drive; that way you can ensure that only the Desktop folder is being synced. This is with the caveat that while your Desktop files are being stored in iCloud, they're still taking up space on your HD before, during, and after sync. If you do it on every Mac then you'll keep your Desktop in sync between them all (files added, deleted, changed etc); this might not be what you're after though.
If symlinking isn't something you know how to do, instructions are below.
Backup (via TimeMachine or whatever you use).
Then fire up Terminal and run the following to move your Desktop folder into your iCloud Drive:
mv ~/Desktop /Users/<yourusername>/Library/Mobile\ Documents/com~apple~CloudDocs/
Then just symlink it to the usual 'Desktop' location:
ln -s /Users/<yourusername>/Library/Mobile\ Documents/com~apple~CloudDocs/Desktop /Users/<yourusername>/Desktop
Now anything that you add to ~/Desktop (by dropping things onto your desktop etc) will actually be dropped into your iCloud drive. They are of course still taking up local space on your machine. When you delete something from your desktop then it's gone from the 'Cloud' as well. The benefits of this are more for sync between machines (+ access via your iPhone of course).
If you've got another mac, move anything you value from it's desktop folder into your iCloud Drive 'Desktop' folder first, then delete ~/Desktop on that machine and symlink again as above.
Best Answer
Put your large video files into Downloads. This is what I do for instructional videos from Lynda Don't sync or use iCloud Drive for Downloads