iTween is no longer included by default in PlayMaker 1.9 or higher. It can be imported from the Add-ons Manager.
IMPORTANT: You will need to import iTween from the Asset Store BEFORE importing the Add-on.
NOTE: iTween now requires Unity 5.6.4 or higher. If you are using an older version of Unity you can download the old version of iTween that was distributed with PlayMaker:
More info on iTween: http://itween.pixelplacement.com/index.php