By default, HTML links can be added to the plain text window/WYSIWYG window below the guest book section of edit page.  However, if you enable the site option for HTML messages, this will allow you to add links to the text box within the main content area.  Please note, that this will only give you the ability to add an actual URL (not the capability of embedding a link into the copy of the text).  To have this site option enabled, please contact support