Download the latest installer from here.
Choose .exe if you are on windows and .jar for Mac or Linux. You should already have Java installed for Minecraft. Simply click install on the window that pops up, then exit once it is Done.
Messages sent from other players will be displayed as PCS images and the Minecraft pause screen will display information on how to connect to your touch device.
This installs Pictocraft along with the following Minecraft modifications: