Select the table row and go to Format | Borders and Shading. Select the type
of border you want and then click in the appropriate location (top or
bottom) of the preview. This is quite unintuitive, as it always seems to me
more natural to select the border position and then choose the type and
weight of border, but that's not the way it works. You can also do this from
the Tables and Borders toolbar. Word remembers the last line style and
weight you use, and you can use the buttons on the Borders palette
(Formatting or Tables and Borders toolbar) to apply additional borders of
that type; the border settings will stay the same until you change them
again.