The plugin page claims to take in the same arguments as wp_nav_menu() but the 'echo' argument doesn't seem to be functional at present.
I was able to get this to work very easily by opening the plugin code and making one tiny change. In dropdown-menus.php, in the dropdown_menu function, I added "return " in front of the line reading "wp_nav_menu( $args );"
This now works with and without the echo argument set on my install. I hope this helps and thanks for a great plugin!