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.
How do they get the deer to cross at the signs? Or to read this tiny ad?