Fixed it, your instructions were wrong.
Here is the new set of instructions:
Admin CP -> Styles and Templates -> Style Manager -> Edit Template -> postbit_legacy.
and
Admin CP -> Styles and Templates -> Style Manager -> Edit Template -> postbit
Code:
<a href="http://digg.com/submit?phase=2&url=http://www.url.net/forum/showthread.php?$session[sessionurl]p=$post[postid]&title=$post[title]" target="_blank" title="Digg it"><img src="$stylevar[imgdir_misc]/digg.gif" border="0"></a>
Regards