If you use the first export option (all users) and get a bunch of errors saying wp_usermeta tables not found, you need to change this on line 150 of users2csv.php:
find:
wp_usermeta
and replace it with:
".$wpdb->prefix."usermeta
The error is caused by the plugin falsely assuming you are using 'wp' as a prefix for your tables. Which is not necessarily true because you can provide your own prefix during installation of Wordpress.