Hi,
You changed the code to show another image (navigationBar.png), but that image is not in the plugin so it will not show up.
You need to upload the image somewhere and change the URL for the image to that, not a relative one like you have now.
So
[code:1eokvofo] background: #999999 url("../multi-level-navigation-plugin/images/navigationBar.png") repeat-x;[/code:1eokvofo]
would become something like this:
[code:1eokvofo] background: #999999 url("http://domain.com/navigationBar.png") repeat-x;[/code:1eokvofo]