Yes, that's right. And, MH only flys from EZE to CPT/JNB a few days a week. Also, the only way you can check WorldPerks availability or make an award travel reservation on MH is by calling Northwest.

So, I'd find out which days Delta has C class award availability using NWA.COM (you might have to start your search with ATL - EZE and then whereever you want to fly from - ATL) and which days MH flys to CPT/JNB using Expedia - then call Northwest and make your reservations.