You can't.
Search Engines do what they want, you can't change that. Make sure there are links to those pages, aside from that theres nothing you can do.
Sometimes using a standard file extension helps though ( .php, .htm, .html )
Also realize that generally search DIRECTORIES are different from SEARCH ENGINES, and most of the time Directories are human-edited, not spidered, and further most of the time directories only display the domainname... not individual URLs.
