Can’t get CSS Into the Head via add_action

You don’t call enqueue or register functions hooking into the wp_head hook. At that hook its too late to add more, I guess, unless you play with priority parameter.

Use the init hook to call wp_enqueue_* & wp_register_* functions.

Do it like this:

<?php
add_action('wp_enqueue_scripts', 'addCSS');
function addCSS() {
    wp_register_style( 'prefix-style', plugins_url('AddToCartStyles.css', __FILE__) );
    wp_enqueue_style( 'prefix-style' );
}
?>

This should work! Just changed the hook in your code.