Please wait for the next 1.5.2 version release. It comes within next 1-2 days. Then add this custom code to disable the rel  = nofollow of all external links:

function remove_nofollow_rel_from_external_links( $do ){
    return false;
}
add_filter( 'wpforo_external_link_nofollow', 'remove_nofollow_rel_from_external_links');

How to Easily Add Custom Code in WordPress (without Breaking Your Site)