I believe you forgot to add quotes around the menu parameter.
<?php $post_meta_value = get_post_meta($post->ID, 'tabmenu', true);
echo do_shortcode('[custommenu menu="'.$post_meta_value.'"]'); ?>
I believe you forgot to add quotes around the menu parameter.
<?php $post_meta_value = get_post_meta($post->ID, 'tabmenu', true);
echo do_shortcode('[custommenu menu="'.$post_meta_value.'"]'); ?>