If you just want an executable jar for your Slick2D application then you can use JarSplice
to do this for you.
Export your project classes and resources (images, sound, etc) into a normal non runnable jar. Then run JarSplice, on the jar tab add your jar and all the external libraries you are using (lwjgl.jar, slick.jar, etc), add all the natives you are using on the native tabs, specify your main class and click to create an executable jar.
Additionally you can also use it to create platform specific executables (i.e. windows .exe files, OS X .app files, etc).