I think there is a little misunderstanding here! The Lightweight Sidebar Manager plugin allows you to build sidebars and place them in a location set by the theme. We have answered a frequently asked question in the description stating that the plugin works with all the themes that have sidebar locations defined.
Could you please confirm whether your client’s theme has a sidebar defined within it? If yes, and things aren’t still working for you, we’ll need to take a closer look to see what is happening.
If you could open a ticket at our support center, we can help you get things working. 🙂