If you want to adjust cells individually, I think what you're looking for is cell properties. If you notice in Susan's example she was setting the margins for an entire row. Instead, choose the cell options. You'll need to drill down to the Options and deselect Same as the whole table.
If you add larger margins than your row height or column width has room for, you'll have to adjust the size of your cell to accomodate that. Cell width can be defined in the Cell page of the Table Properties dialog box. It's also possible to do this with your mouse. For instance, to move the "wall" between adjacent cells in the same row, select those two cells then use the double-headed arrow to resize the cells. I've found variable row heights can be acheived by splitting cells then re-merging cells in a different configuration.
Keep Up with TechRepublic