I merged Dude's code into the first answer on this post to make it more clear http://www.kriesi.at/support/topic/customise-social-stuff#post-59026
But you can open up the file sidebar.php and find line 79
**You need to update your theme, current version is 1.7 and you have 1.3. Please go to themeforest.com with the account you made the purchase with and download an update. You will need to use FTP to update the theme which will overwrite all the template files, so if you edited any theme filed , please make a backup first. Otherwise the lines wont match up. I made a video explaining the process if you have modifications you want to save https://docs.google.com/file/d/0B8hqGBMSfHtKRkJlN0pyNVNDSkk/edit?usp=sharing.
and on line 79 (which is empty), please paste this
echo "<li class='text-sep'>/</li><li class='twitter'><a href='http://YOUR-URL/'>Tumblr</a></li>";
To make sure you in the right place, line 84 looks like
echo " </ul><!-- end social_bookmarks-->";
Just you will need to change the YOUR-URL to whatever the url you want to display for tumblr, I really don't use them so not too sure. But if you get stuck, please let us know.