It would be easier if you used dried egg white instead of meringue powder, which also includes sugar, gum, starch, and a whole lot of other ingredients (in very small quantities). But in the case of frosting, it's not going to matter much whether you use meringue powder or dried egg white. The amount of sugar in the meringue powder is not going make an appreciable difference in a recipe that's already mostly sugar. Either way, you get the benefit of pasteurization, which saves you from the potential risk of eating uncooked egg white.

If all brands of meringue powder and dried egg white work the same way (check your package for confirmation), mix 2 teaspoons of meringue powder with 2 tablespoons of warm water for each egg white you need. Make sure the powder is completely dissolved (sometimes it tends to clump). Then mix the solution into the recipe according to the instructions.