Make sure that the scripts and styles for the Multi Event Registration add-on cart are loaded on tribe_events event pages.

mer_scripts_everywhere.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function my_add_mer_scripts_everywhere() {
        wp_enqueue_style(
            'espresso_multi_event_registration',
            apply_filters(
                'FHEE__EED_Multi_Event_Registration__enqueue_scripts__event_cart_css',
                EE_MER_URL . 'css' . DS . 'multi_event_registration.css'
            )
        );
        wp_enqueue_script( 
            'espresso_multi_event_registration', 
            EE_MER_URL . 'scripts' . DS . 'multi_event_registration.js', 
            array( 'espresso_core' ), 
            EE_MER_VERSION, true 
        );
}
add_action( 'wp_enqueue_scripts', 'my_add_mer_scripts_everywhere', 11 );