At Admin Panel -> Settings -> Switches, you have a "hits in" option. Hits in means visitors coming to your site from one of the sites in your directory, via a link they've placed to you.

Hits in can be counted in two ways:
1) If they click from the referring site directly to your directory index page, the source will be automatically detected based on HTTP_REFERRER headers.
2) If you want them to land on your homepage which is outside of WSN, but still be counted, you need to have them use a special in.php?id={LINKID} link to link to you. This will associate the hit with the proper link and show the visitor your homepage (as set in Admin -> Settings -> System Configuration -> Website Details).

To guide people on how to link to you, customize the "Linking and voting code" template at Admin Panel -> Themes -> Manage Templates.

Be sure to update the outgoing emails to reflect the method you wish to use. Some people dislike method 2 since it may not be good for SEO purposes (the 301 redirect in in.php only transfers a percentage of the link juice)... also it requires they wait until after submitting their link to know what linkback URL to use.

If you want people who click the in.php link to be directed to the link's forum instead of the front page, use in.php?id={LINKID}&destination=forum. If you want people to land on the link details page, use in.php?id={LINKID}&destination=detail. The latter could be desirable if submitters are going to want to brag about having a topic in your directory.