Can anyone explain to me how to make meringue buttercream. I see it used as a filling but can't seem to find a recipe for it. Thanks

There are several versions, basically starting with a egg base and adding butter.

Swiss Meringue Buttercream- SMBC -only the eggwhites are used and cooked with the sugar
Italian Meringue BC IMBC The egg whites are whipped and a boiling syrup is added (can cause some crystalization)

Duff Goldberg uses a version that does not require 'cooking' but you must use pasteurized egg whites: http://www.foodnetwork.com/recipes/food-network-challenge/french-buttercream-recipe/index.html

Bring to a boil: 1 - 1/2 pounds of sugar and 1 - 1/2 cups cold water. Continue to boil approx. 10 minutes until it is thick and has slow bubbles. meanwhile, whip 9 egg whites to medium peak. With mixer still running, slowly pour the syrup in. Continue to whip until the bowl feels warm, not hot. Now, add 2 - 1/2 pounds of butter that has been cut up into chunks. Add 2 tsp. Vanilla extract. That's it - you're done! You can flavor it with melted chocolate, no problem.