The easiest way (at least how I look at it) to do so in DW is to the enter code view mode (( View>Code View)) and manually type them in. You can also use the objects palette (( defaults to "Common" - click on it, then "Head"; then select the iconn of the one you want :: Keyword ~or~ Description )) to do so...personally I like more control over where it's placed.
Meta "Keyword", Meta "Description", and proper use of HTML tags will generally do the most for placement. Yahoo! and Google are for the most part more interested in the actual content of the page than they are in the meta info the author provides. Some suggestions: Don't go overboard on the Meta information, be concise and to the point for each page (specific to the content on each page). Use a proper title that accurately describes the content, use the h1 similarly (only once per page) and the remaining hn tags as needed and within spec.