Moving entire folders
To move a folder from one channel to another, simply drag the folder icon over to a new channel on the left-hand side of the screen. Make sure you drag the folder icon itself, otherwise you'll only be able to re-order the folders within the source channel.
Drag the folder icon to a new channel in the Channel List. The destination channel will turn green. Let go of the folder, and it'll drop into its new home!
After any drag-and-drop operation, you'll have 10 seconds to undo the move if you change your mind - just click the "Undo" link in the bar that appears at the top of the folder list.
Moving individual files
If you just want to move one file instead of a whole folder, you can perform the same drag-and-drop operation within the folder itself. Just grab the particular slideshow or playlist you want to move and drop it into a new folder.