Eclipse plugin cannot find class from class folder

I'm developing an Eclipse plugin, and want to use some pre-compiled classes which I've put to `lib/` folder in my project (so the directory structure looks like `{proj_root}/lib/com/example/Required.class`). Then I've added `lib/` folder as class folder to project's build path. At build time all is working fine, I can import and use that classes in my code. And now the problem is, when I'm running my plugin in separate Eclipse instance I get `ClassNotFoundException` by my plugin refering to class from precompiled classes folder. I tried specifying my class folder in "Order and export" tab in Build path settings. Also I tried setting checkbox in plugin's build.properties GUI editor in front of `lib/` folder to be included in binary build. In that case if I export my plugin to JAR I can see `lib/` folder inside it, with my classfiles, but at runtime my plugin still cannot find any of classes from `lib/`. Can you please give me an advice on how I should tell Eclipse where to find that classes at runtime? PS.: - I did research on stackoverflow on this problem, but all suggestions are the same: just add class folder in project's Build path. - At this point in plugin's build.properties Text editor, eclipse shows warning `The class folder 'lib/' is not associated to any output library entry.` and suggests to make this modification of code: `output.. = bin/,lib/`. I have tried that solution but it is seems to change nothing with the problem.