They should come out Dec 11th-Feb 24th at 5pm-8am. You only get 2 snowballs a day and make sure you don't make a snowman at all, otherwise you won't be able to get a Dung Beetle. There will only be 1 out at those times so keep watch over your snowballs.
You get 2-3 snowballs a day. Sometimes it's 3. Other times it's only 2. But I've found something really convenient about dung beetles. If one is on a snowball, and you attempt to get it but it flies away, just go in a building. Or walk away for a few minutes. Come back, and a new dung beetle should be there. Hope that helps.