Enchanted christmas pudding are found inside various pots and sacks that you can "check" in the Wizard's Tower kitchen during the 2011 Christmas event. If you watch these, you'll eventually see one hopping inside one of the pots or sacks. This means that there is an enchanted christmas pudding in it. "Check" the pot or sack with the hopping enchanted christmas pudding to obtain one enchanted christmas pudding.
Players from the United States may be confused by appearance of the christmas pudding. While in the United States pudding chiefly refers to a custard-like milk based dessert, in the United Kingdom (where Jagex is based) pudding can also refer to any rich starch-like or dairy based dessert, and that matches the description of an enchanted christmas pudding found in Runescape.