You shoud enter this code in "view_video.tpl" file which is located in templates folder.

In this file you should locate the following line:

<a href="http://myweb2.search.yahoo.com/myres...fo.video_title}" target="_blank" title="My Yahoo!"><img src="{$img_css_url}/images/yahoo.png" border="0" alt="yahoo" /></a>
and then add below this line:

&nbsp;
<a href="Twitter}" target="_blank" title="Twitter"><img src="{$img_css_url}/images/twitter.gif" title="Follow me on Twitter" width="18" height="18" border="0" alt="Follow me on Twitter" /></a>
I hope this will help.

Thanks