Trackbacks and pingbacks from low pagerank sites or blogs can eat your pagerank juice. Trackbacks looks good as this says someone has liked you post, but this can be harmful for your website importance, as you are giving link to them.
Some themes in wordpress can have rel="nofollow" added to the trackback links and some may not. You should check for trackback links. Links to good sites can help you in better SEO but links to less important sites from your blog can eat your SEO points. Maintaining each link may require you use a plugin which provide you editing facility to each trackback comments. So, better to add rel="nofollow" to all the trackbacks. After that trackbacks will be just a good information to you and your visitors. but it will not affect your SEO.
You may need to edit your comments.php file. This file may not be inside your theme's template folder. In this case you need to use ftp or cpanel to modify the file. Inside template folder comments.php file may be just using other comment file. In this case, even if you see comments.php file inside your template folder, you cannot modify your trackback links code from wordpress admin panel.
Use any ftp client (FileZilla, gftp) or cpanel and go to your theme folder. Find comments.php file and search link for trackbacks.
Here is my comments.php code:
<div class="title"> <a href="<?php comment_author_url() ?>" rel="external nofollow"> <?php comment_author(); ?>> </a> </div>
and, In my comments.php file, I have external nofollow added. If you are using internal links as trackbacks also then you need to remove the external keyword. Just nofollow is enough and important.