To add a link to an entry, first select that entry in the Sidebar. Next, place the insertion point in the location at which you want to insert the link. If you want to add the link to some pre-existing text, select that text. Next, open the Edit menu in the menu bar and then move down to the Insert submenu. Choose the "Link" item in that menu. A sheet will drop down and allow you to edit the text that will be visible and the URL of the link. This URL can be anything that you want, such as a web page. Next to the URL field, there are two buttons that help you make links. The first button will let you choose any other journal or entry in MacJournal to which to link. The other button will let you select a file to which to link.
After you are finished editing the link, click the OK button accept the change. Cancel will discard any changes you made. The Remove button will remove any pre-existing link that you are editing.