Solution found! Microsoft helped me to figure out that my new mac computer was NOT hooked up to a printer yet. So that is why the margin-related tabs/dialog boxes would not open. The Microsoft technician, a wonder, patient person named Justina in Nigeria helped me to go into my Printer Setup dialog and reset everything. This worked. So now, even though I don't have an actual printer plugged into my Mac computer, I can access the custom margins and Format/Document tabs to work on my margins. This took 3 hours Microsoft to figure out!