You have two options to play the full version of the Learn to Write with Mr. Pencil app:

Use the Mr. Pencil stylus to unlock the full version. Press the Unlock button at the Home screen. Make sure the eraser end of the stylus is laying flat in the circles as they appear on screen. You will receive a star flourish if the game registers the eraser.

Make an in-app purchase to unlock the full version. Select Try Me from the Home screen, and when you get to the main menu, there is an Options icon in the lower right hand corner (it is the "?" symbol). The ability to purchase the full game is located in that menu. You will need to log-in to your iTunes account to make the purchase.