Adding Adjuncts to Meringues
You want to be sure that the egg whites are fully whipped before you add pieces of anything (it's also best to incorporate sugar as either a syrup or very finely ground sugar once the eggs are already whipped as sugar can make the whipping take longer).

When you incorporate pieces of anything into your meringue, be it nuts, candy, etc, fold it in gently with a spatula. You can also add a stabilizer like cream of tartar to help prevent the meringue from falling.