You don't want to use Meringue Powder to make a cake mix rise -- Meringue powder is powdered egg whites with stablizers which can alter the taste of the cake mix. The stablizers are put there to suspend the liquid molecules thus making things like royal icing dry hard (because the liquid molecules cannot move they cannot remain fluid.) This can also slightly alter the texture of your cake as well.
You are better off using 2 TBSP powdered egg whites per box of cake mix -- this gives you volume without added richness and no weird aftertaste. Hope that answers your question! 
I guess it's whatever works for you. I'm sharing my experience. Plus I love science and I have to know how every little thing works!
Yes, add the powdered egg whites IN ADDITION to the egg whites you will be using already. Using the powdered egg whites will give you the volume without adding extra liquid to the recipe (which can alter the final outcome...
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About Wilton's suggestion of using the meringue powder, remember Wilton is there to teach AND sell and they have their own brand of meringue powder. Just be aware of that.
Thanks Bob for the info. I too put mp in my cake mixes. Maybe that's why my chocolate cake went bad on me.
Well it could be, it's hard to say. I know some people who swear by it -- I'm not one of them! 
I'm sharing my experience with y'all and I personally find the powdered egg whites to give the best overall performance with no aftertaste or disruption to the final product. But as I always say, do whatever rocks your world! 
MP in the cake? 
Must depend on the Wilton instructor cause mine didn't suggest that.
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