Many thanks, you are really helping clarify!
I've added the call to ninja forms in a child theme functions.php file
Not sure what you mean though by //do whatever you need to do, using $formID.
Is this where I would place my code, and does $formID need to be defined?
add_action('ninja_forms_display_fields', 'thisIsMyFunction', 0, 10);
function thisIsMyFunction($formID){
//do whatever you need to do, using $formID
global $current_user;
get_currentuserinfo();
echo $current_user->display_name . "\n";
echo $current_user->pmpro_baddress1 . " ";
echo $current_user->pmpro_baddress2 . "\n";
echo $current_user->pmpro_bcity . ", ";
echo $current_user->pmpro_bstate . " ";
echo $current_user->pmpro_bzipcode . "\n";
echo $current_user->user_email . "\n";
echo 'Social Security Number: ' . $current_user->SSN. "\n"; ?>
};