Lee - I'm not sure whether you wanted the entire cell
(edge to edge) or most of the cell, so here are two
answers.

You want to use borders not underline.  For edge to edge
or most of the cell the steps are almost identical.

From the Format menu, choose Borders and Shading.
On the Borders tab select your line style, width, color,
etc.
In the Preview diagram click the bottom border button.
If you want edge to edge, from the "Apply to" drop-down,
select Cell,
 or
If you want most of the cell, select Paragraph.
Click OK.

That should do it.

Hope this helps.