Just type them in under your ad. You can also put them in the body of the ad. You can have as many pages as you want as long as they are different. Each page you have will show up in the search results depending on the search terms searched for and contained in your listing. The search works by searching for words contained in the ad. In my ad at http://www.galesburgpages.com/node/20 I just placed them at the bottom, this is the easiest way. Some web sites do this and call it a tag cloud. These are just keywords to get the web page found.