Sorry for the delayed response. There's a few ways to do this, and it depends on how you have Kunena setup to handle registrations in the Integration tab. However, I'll give one example, and hopefully that will help. If it's not the specific area you're looking for, and you need help doing it elsewhere, just let us know.
To add the Login With Facebook to the little header area of Kunena where it says "Welcome, Guest" and shows a horizontal login form, do the following:
1) Edit the file /components/com_kunena/template/default/loginbox/login.php
2) At line 58, add the {JFBCLogin} tag below (I left the 2 lines above it so you can make sure you're at the right spot)
<?php echo JHTML::_ ( 'form.token' ); ?>
</span>
{JFBCLogin}
This will add the Login with Facebook button directly next to the normal Login button. Please note, these instructions are for the default Kunena template, and may be slightly different for a different template, or a modified one.
Again, hope this helps, and we'll add this to our Common Support Questions for profile integration shortly. Let us know if you need anything else!
Alex