Minecraft Crash

---- Minecraft Crash Report ----
// You're mean.

Time: 20/05/13 11:57
Description: Failed to start game

cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraftforge/common/Configuration
	at cpw.mods.fml.common.LoadController.transition(LoadController.java:142)
	at cpw.mods.fml.common.Loader.loadMods(Loader.java:515)
	at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:160)
	at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:310)
	at net.minecraft.client.MinecraftAppletImpl.func_71384_a(SourceFile:56)
	at net.minecraft.client.Minecraft.run(Minecraft.java:607)
	at java.lang.Thread.run(Thread.java:680)
Caused by: java.lang.NoClassDefFoundError: net/minecraftforge/common/Configuration
	at mods.MutantCreatures.MutantCreatures.preInit(MutantCreatures.java:69)
	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 cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:494)
	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.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
	at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
	at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
	at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
	at com.google.common.eventbus.EventBus.post(EventBus.java:267)
	at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
	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.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
	at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
	at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
	at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
	at com.google.common.eventbus.EventBus.post(EventBus.java:267)
	at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
	at cpw.mods.fml.common.Loader.loadMods(Loader.java:514)
	... 5 more
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.common.Configuration
	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:238)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	... 32 more
Caused by: java.lang.NullPointerException
	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:227)
	... 34 more


A detailed walkthrough of the error, its code path and all known details is as follows:
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-- System Details --
Details:
	Minecraft Version: 1.5.2
	Operating System: Mac OS X (x86_64) version 10.6.8
	Java Version: 1.6.0_45, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 471156616 bytes (449 MB) / 534708224 bytes (509 MB) up to 1065025536 bytes (1015 MB)
	JVM Flags: 3 total; -Xbootclasspath/a:/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/Resources/LauncherSupport.jar -Xms512M -Xmx1024M
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: FML is installed
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	FML: MCP v7.51 FML v5.2.10.647 3 mods loaded, 3 mods active
	mcp{7.44} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized
	FML{5.2.10.647} [Forge Mod Loader] (coremods) Unloaded->Constructed->Pre-initialized
	MutantCreatures{1.3.4} [Mutant Creatures] (Mutant Creatures v1.3.4 mc1.5.2.zip) Unloaded->Constructed->Errored
	LWJGL: 2.4.2
	OpenGL: ATI Radeon HD 4670 OpenGL Engine GL version 2.1 ATI-1.6.36, ATI Technologies Inc.
	Is Modded: Definitely; Client brand changed to 'fml'
	Type: Client (map_client.txt)
	Texture Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null