How do you alter margins using Microsoft Word on a mac?

It should be the same on the Mac version and the Windows version of Microsoft Word. If the ruler is displayed on the top and at the side of the document window, you can use it to change your margins. Hover your mouse over the the separator between the blue area and the grey/white area of the ruler (you may need to move the indent controls out of the way first -- you can move them back later). Click and drag this either direction to change the margin. Also, if you hold down the option key, you can see the lengths between the margins (i.e. the distance from the edge of the document to the margin). If you need to adjust the margins more precisely, you can use the Document formatting dialog. It's under the menu "Format" > "Document..."