Pie Register Plugin. Where exactly to add code?

Whats up members. I install the pie register plugin, and when I go to change the settings for it, it tells me to add <?php
session_start(); ?> to the top of my wp-login.php for it to work properly. I know where it’s at, and how to edit it however im not sure exactly where to put it. Ive tied manyplaces at the top but when i did it juz messed the page up so i would change the code back to it’s original version. If anyone has been through the same thing or know how to fix it I will greatly appreciate it