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.