<i><b>Added later: this is resolved in the new release, v1.7.5 – just upgrade or download the new plugin version and it’s all good again.</b> (Removed attachment – unnecessary now)</i>
Sorry to be the bearer of silly news… It looks like SVN pulled another stunt: the two Scripts and Images directories weren’t packaged in the v1.7.4 release of the Multi-level Navigation Plugin. 
For Multi-level Navigation Plugin users of "standard" CSS for the menus, who after upgrading now unexpectedly miss the menu backgrounds and/or find that extra jQuery (extended) options don’t work, it’s no biggie to resolve: if you upgraded, just reupload the two folders. I hope I’m not overstepping my bounds here, <s>but I’ve included a .zip file to this post with the two skipped directories – unzip, and</s> plop the two directories <b>inside</b> the plugin folder, so you end up with something like this:
../wp-content/plugins/multi-level-navigation-plugin/images
../wp-content/plugins/multi-level-navigation-plugin/scripts
Done. 
Silly that SVN doesn’t have a better method to avoid these little sneaky mishaps of not including files that haven’t changed…