In a double boiler, melt the candy bark. If you don't have a double boiler (and I don't), place a bowl over a pan with a small amount of water. Make sure that no steam escapes around the edges of the pan.
After the chocolate has finished melting, add the cinnamon and the nutmeg.
Now stir in the rum. It's going to get ugly, but hang in there with me.
Quickly stir in your eggnog, going just a little at a time until the mixture smoothes. You may not need to use all of it.
Let it cook a little bit longer. This will make it smooth and make sure that any extra moisture cooks off.
Pour over the cereal and stir, making sure that it coats the cereal as evenly as possible.
Add the powdered sugar until the cereal is completely coated.
Pour the puppy chow out over wax paper so that it is in a single layer. Let it cool completely, about an hour or two, before storing.