You could also create at the project root the following create_jar.xml
file:

<project name="create_jar" default="create_jar">
    <target name="create_jar">
        <mkdir dir="jar" />
       <jar destfile="jar/myFile.jar">
           <fileset dir="bin" />
           <manifest>
               <attribute name="Main-Class" value="MyClass" />
               <attribute name="Class-Path" value="externalFile1.jar
externalFile2.jar" />
           </manifest>
       </jar>
      </target>
</project>

(This will create the jar/myFile.jar jar file
with the files located in bin sub-folder,
with MyClass as main class,
with externalFile1.jar and externalFile2.jar as jars in the search
class path).

Of course, you must change the values appropriately for your project.

When you're ready to create the jar file, simply right-click on the xml
file
in the package explorer window, then select 'Run As' -> '1 Ant Build'

Regards