Wikipedia+ uses a list of sites to search for alternative perspectives on the Wikipedia page. The default set of sites is easily customisable by removing/adding other sites.
The options dialog has a Websites pane which lists the sites to consult for page suggestions. The higher the site is up the list, the earlier it is consulted for a page suggestion, so put your favourite sites highest on the list.
Adding a New Website
To add a website, click on the plus icon, and an empty dialog will appear to specify the new site's details, as shown on the right. You must give it a name and an access URL; all other properties are optional. The 'icon' field is for the URL of the sites favicon, and the 'shortcut key' refers to the letter which is underlined when the site suggestion appears on the notification. Maybe the easiest way to fill in this information is to browse to the site of interest and then click the "Use current" button, which will fill in the software's best guess at the details appropriate to that site.
Removing a Website
To remove a website, first select it by clicking on it then click on the orange cross icon.
Editing an existing Website
Most users will probably not need to edit sites much, though webmasters may find it helpful if they are enabling their site for Wikipedia+. To edit a website, either select it and click on the 'Edit' button, or just double click on it. This brings up the dialog shown on the right, in which the website's properties can be changed.