Essentials do not work

[apiocera]

Throws exception on startup:

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:175)
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:215)
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:136)
    at net.glowstone.GlowServer.loadPlugins(GlowServer.java:508)
    at net.glowstone.GlowServer.start(GlowServer.java:419)
    at net.glowstone.GlowServer.main(GlowServer.java:109)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/server/Packet
    at com.earth2me.essentials.Essentials.<init>(Essentials.java:52)
    ... 15 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.server.Packet
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:36)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:24)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    ... 16 more