I try to do it the following way:
Right click on the name of the project.
Click on Build Path in the drop dawn menu.
Click on "Configure Build Path"
And then I do not know what to do. Should I select "Source", "Projects", "Libraries", "Order and Export"?
In "Libraries" I have "Add JARs..." and "Add External JARs...". What should I select? (I have already a .jar file in the lib folder of my project.)
ADDED:
If I click on "Add JARs" in the "Libraries" tab, I see the "lib" sub-folder but if I go there I do not see my .jar file there (and I know that it is there).