Navigation Arrows

to have side navigation with next prev arrows only in one category you have to place in your child-theme functions.php :

add_filter('avia_post_nav_settings','avia_same_category_filter', 10, 1);
function avia_same_category_filter($settings) {
     $settings['same_category'] = true;
     return $settings;
}

why you are not seeing them now i cannot say because i do not see the link to your site.
Maybe you are using on that post a fullwidth slider – for that case you can use in child-theme functions.php :

add_filter('avia_post_nav_settings','avia_remove_fullwidth_slider_check', 10, 1);
function avia_remove_fullwidth_slider_check($settings)
{
$settings['is_fullwidth'] = false;
return $settings;
}

if you want to change the directions ( for me it is more logically when going to the right side it comes next entry ) :

add_filter( 'avia_post_nav_entries', 'enfold_customization_postnav', 10, 2); 
function enfold_customization_postnav($entries, $settings) {
    $entries['prev'] = get_next_post($settings['same_category']);
    $entries['next'] = get_previous_post($settings['same_category']);
    return $entries;
}

by the way – this snippet should be part of the enfold documentation – because the question occurs frequently

and btw 2 : the theme demo theme options download should be updated – only 10 of 26 are available yet