extract the zipfile using winzip.

Read the "README.txt" in javamail-1.4 directory

Use any unzip program to extract the files into a directory. On my
Windows XP system Sun chose to install the JDK and JRE in a
sub-directory of C:\Program Files\Java so that is where I put my
libraries. I will assume that your unzipped files ended up in
C:\Program Files\Java\javamail-1.4.

Now open NetBeans and use the menu Tools->Library Manager and click
the "New Library" button. Choose a name for the Library; I chose
JavaMail. Leave the Library Type as "Class Libraries". In the
Classpath tab press "Add Jar/Folder" and enter the full path of
mail.jar. In my case it is C:\Program
Files\Java\javamail-1.4\mail.jar. You don't have the source code so
ignore that tab, click Javadoc and enter the full path to the library
Javadocs. In my case it is C:\Program
Files\Java\javamail-1.4\docs\javadocs.

To complete the task you must add the JavaMail library to your project
so open your project, right click on Libraries, choose Add Library,
select JavaMail and you're done.

Well no, you have not finished :( JavaMail relies on the Java
Activation Framework so you need to repeat the above procedure
starting with the download of the Activation Framework from
<http://java.sun.com/products/javabeans/jaf/downloads/>.

You could add the Activation Framework jar and Javadocs to the Java
Mail library but I think it is preferable to keep them separate in
case you want to use the Activation Framework in a project that does
not need JavaMail.