Which forum? If you are talking about your one or one you are working on, you can find the HTML in a "templates" or similarly named directory. If you can be more specific, e.g. telling us which forum you are using, then I can be of more help.
In the themes/*/*.template.php files then. The right one depends on where you wanted the links. This is assuming you want static links on the header or footer somewhere. Of course, you might be wanting something else, e.g. links for each post, I don't use social bookmarking or know much about it hence the reason I don't know exactly what you are trying to do.