How to add an existing smart folder to the sidebar
In Finder, select your Smart Folder (typically saved in ~/Library/Saved Searches, it's a .savedSearch file).

Then from the menu in Finder, select: File > Add to Sidebar &nbsp;&nbsp;&nbsp;^⌘T

Or after selecting the Smart Folder file, just press ^⌘T on the keyboard.

Note that in OS X 10.8 and earlier the keyboard short cut was: ⌘T