Many have issues setting up a mission so here is a basic one I use to test usually.  Note that for me it does not work on creative server, only adventure.  I have circled red the items I entered.  the item id is my mission ruler, so when I examine it, it spawns 1 acorn in my inventory.  The mission will repeat over and over as many times as you want to examine.  (you create missions with the 'mission ruler' you can spawn one with an ebony wand)     In this screenshot there is an 'x' in