Pixelmon 2.5.5 crash for Minecraft 1.6.4?

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 12/3/13 4:27 PM
Description: There was a severe problem during mod loading that has caused the game to fail

cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: cpw/mods/fml/common/Mod$Item
at cpw.mods.fml.common.LoadController.trans…
at cpw.mods.fml.common.Loader.initializeMod…
at cpw.mods.fml.client.FMLClientHandler.fin…
at net.minecraft.client.Minecraft.func_7138…
at net.minecraft.client.Minecraft.func_9999…
at net.minecraft.client.main.Main.main(Sour…
at sun.reflect.NativeMethodAccessorImpl.inv… Method)
at sun.reflect.NativeMethodAccessorImpl.inv… Source)
at sun.reflect.DelegatingMethodAccessorImpl… Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launc…
at net.minecraft.launchwrapper.Launch.main(…
Caused by: java.lang.NoClassDefFoundError: cpw/mods/fml/common/Mod$Item
at pixelmon.config.PixelmonItemsPokeballs.a…
at pixelmon.config.PixelmonItems.addNames(P…
at pixelmon.config.PixelmonConfig.loadConfi…
at pixelmon.Pixelmon.init(Pixelmon.java:104…
at sun.reflect.NativeMethodAccessorImpl.inv… Method)
at sun.reflect.NativeMethodAccessorImpl.inv… Source)
at sun.reflect.DelegatingMethodAccessorImpl… Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at cpw.mods.fml.common.FMLModContainer.hand…
at sun.reflect.NativeMethodAccessorImpl.inv… Method)
at sun.reflect.NativeMethodAccessorImpl.inv… Source)
at sun.reflect.DelegatingMethodAccessorImpl… Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler…
at com.google.common.eventbus.SynchronizedE…
at com.google.common.eventbus.EventBus.disp…
at com.google.common.eventbus.EventBus.disp…
at com.google.common.eventbus.EventBus.post…
at cpw.mods.fml.common.LoadController.sendE…
at cpw.mods.fml.common.LoadController.propo…
at sun.reflect.NativeMethodAccessorImpl.inv… Method)
at sun.reflect.NativeMethodAccessorImpl.inv… Source)
at sun.reflect.DelegatingMethodAccessorImpl… Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler…
at com.google.common.eventbus.SynchronizedE…
at com.google.common.eventbus.EventBus.disp…
at com.google.common.eventbus.EventBus.disp…
at com.google.common.eventbus.EventBus.post…
at cpw.mods.fml.common.LoadController.distr…
at cpw.mods.fml.common.Loader.initializeMod…
… 10 more
Caused by: java.lang.ClassNotFoundException: cpw.mods.fml.common.Mod$Item
at net.minecraft.launchwrapper.LaunchClassL…
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
… 41 more
Caused by: java.lang.NullPointerException
at net.minecraft.launchwrapper.LaunchClassL…
… 43 more

Try reinstalling the mod. If that doesn't work, try reinstalling Forge. If all else fails, post a topic on Forge's support forums.