To open files you've saved to iCloud, press Command-O in an iCloud-savvy application, then click on the iCloud button. You'll see something like this:

Note that in the above screenshot you see a folder. To create a folder, just drag one file on top of another, as you would with icons on an iPhone or iPad. Name the folder, and it'll be saved on iCloud.

You may have a number of files on your Mac that you'd like to put in the cloud; this is straightforward. Just open a file with an application that can put documents on iCloud, choose , then choose from the menu. If you want to move a file from the cloud to your Mac, click on the menu and find the folder where you want to place the file. If the folder where you want to move the file isn't in the menu, choose from the bottom of the menu, and navigate to the location you want.