At Admin Panel -> Settings -> Switches you have a "related listings" option. When this is switched on, a related listings field appears on the submit listing and edit listing pages in the admin options area. Only administrators can set related listings, and must do so by writing out a comma-separated list of the the listing id numbers. You can find listing id numbers in the URL of the edit listing page for a listing, as the id= parameter.

Please note the distinction between this option and the "related URLs" option, which is available to all submitters (not just admins) and allows specifying websites that aren't in the directory.

When you've specified related listings for a listing, the listing details page will display the list of related listings. To customize the display in WSN 9+, go to Admin -> Listings -> Listing Fields and edit the "related listings" field... find the "custom display html" area.