Eclipse sharing a group of classes between projects

I have a small Java application with a few classes. I used Eclipse and created a new "project" and put the classes in the default package.

Then I decided a couple of the classes would be useful in other of my applications so I created a new "project" and created a package "". I copied the reusable classes here and deleted them from the original project.

I tried various settings in "Project" "Properties" but cant get the application to compile and run now.

In "Project", "Properties", "Libraries" I used the "Add Class Folder" and ticked the box for "". The app comiles now but at runtime I get "NoClassDefFoundError".

I've tried a few other things such as adding "" as a library and inserting an import statement but am not getting any joy.

Is there an idiots guide to this?