Netbeans doesn't care (and doesn't need) your jar. What you have to do is add the jar to your project, instead of adding an external library to netbeans.
If your project is a maven project (unlikely), add the jar as a dependency.
If your project is a java project then look for a library (lib) folder.
If you are trying to modify the look and feel of netbeans itself: Force look and feel on NetBeans 6.5