Then you probably don’t use a modern theme which makes use of the function comment_form() to create the comment form. Check if you can find this function used in comments.php or a similar file of your theme. If you do and it still doesn’t work, tell me. Otherwise chances are that you do have to add the checkbox manually. Use this function:
<?php if ( function_exists( 'avapr_get_avatar_checkbox' ) ) echo avapr_get_avatar_checkbox(); ?>
