tools->configure->libraries and add the jar as a library. Then for your project add the library through project->properties->required libraries.
You can in the configure libraries screen add libraries at several levels (project, user, JBuilder) so you could add several versions of a library using the same name to different projects. Personally I always add them at user or JBuilder level and give them version numbers if needed. Far easier to avoid mistakes that way.
subject: jbuilder how to add a jar file in class path