How do I disply an array inside a custom post type?

You got ARRAY echos because you saved array on post meta,

Just check array values of meta data like:

print_r( get_post_meta($post->ID, 'ptn_systeem', true) );

and echo it like:

foreach( (array) get_post_meta($post->ID, 'ptn_systeem', true) as  $option):

    echo $option . "<br>";

endforeach;