Give the new library a useful, descriptive short name. This will be used for a filename, so stick with characters which are legal on your platform's filesystem.

Click 'Add' and use the file browser dialog to find the new jar, or if you have unpacked the jar into a directory structure, find the topmost classes directory. Select this jar or directory and add it to the library's classes list.
Click 'Source', 'Add' and find the top of the source path, if your 3rd party library provided source.
Do the same for the Docs tab.

You now have a library definition for your 3rd party jar. Click 'ok' through all the dialogs and you should end up with the new library listed in your project's "required libraries" tab.

JBuilder will automatically insert the paths you defined in the library definition to your project's runtime classpath. The debugger will access the sourcepath to find the java source when you step into a library class. If you see decompiled source instead of the actual source, then the source path is probably incorrect or missing.
2001-Mar-17 12:46am gyles19@visi.com

  <!--JBuilder Library Definition File-->
  fullnameRegex/fullname
  class
    path[../../../JBuilder9/jdk1.4/jre/lib/ext/jakarta-regexp-1.3.jar]/path
  /class
  source
    path[../../../JBuilder9/jdk1.4/jre/lib/ext/src/jakarta-regexp-1.3.zip]/path
  /source
  documentation
    path[../../../JBuilder9/jdk1.4/jre/lib/ext/src/jakarta-regexp-1.3.zip]/path
  /documentation
  lastmodsaved1063507583015/lastmodsaved