Where exactely do you want to put it?
Right beside the "UserCP" and so on links?
If so head to AdminCP -> Style Manager -> Edit Templates (of the style you want to edit) -> locate and double click the "navbar" template (It's in a group also called something with "navblah blah") -> find:
<!-- nav buttons bar -->
Look through the table definition and on a place you find fitting add:
<td class="vbmenu_control"><A href="/diarycp.php.php">Diary</a></td>
There ya go :)