I began with a fresh install of minecraft (with exception of saves, screenshots, server.dat and launcher_profiles.json) DayZ is the only mod installed. I tried an additional fresh install with no luck. The only thing I can think is maybe my version of Java or that I use MacOSX. It works fine as a server or client on my PC's with the same jar file and setup as my Mac. (but they are slower so I want to use it on my Mac) Can you make sense of the crash report?

---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!

Time: 12/22/13 1:57 PM
Description: There was a severe problem during mod loading that has caused the game to fail

cpw.mods.fml.common.LoaderException: java.lang.ClassNotFoundException: com.github.dayzminecraft.dayzminecraft.DayZ
at cpw.mods.fml.common.LoadController.transition(LoadController.java:156)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:521)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:183)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:473)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:808)
at net.minecraft.client.main.Main.main(SourceFile:101)
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 net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Caused by: java.lang.ClassNotFoundException: com.github.dayzminecraft.dayzminecraft.DayZ
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:186)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at cpw.mods.fml.common.ModClassLoader.loadClass(ModClassLoader.java:61)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:249)
at cpw.mods.fml.common.FMLModContainer.constructMod(FMLModContainer.java:462)
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:313)
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.sendEventToModContainer(LoadController.java:201)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:181)
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:313)
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:112)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:511)
... 10 more
Caused by: java.lang.UnsupportedClassVersionError: com/github/dayzminecraft/dayzminecraft/DayZ : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:178)
... 38 more

A detailed walkthrough of the error, its code path and all known details is as follows:

-- System Details --
Details:
Minecraft Version: 1.6.4
Operating System: Mac OS X (x86_64) version 10.7.5
Java Version: 1.6.0_51, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 37687480 bytes (35 MB) / 119709696 bytes (114 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 1 total; -Xmx1G
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Suspicious classes: FML and Forge are installed
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v8.11 FML v6.4.45.953 Minecraft Forge 9.11.1.953 4 mods loaded, 4 mods active
mcp{8.09} Minecraft Coder Pack Unloaded->Constructed
FML{6.4.45.953} Forge Mod Loader Unloaded->Constructed
Forge{9.11.1.953} Minecraft Forge Unloaded->Constructed
DayZ{8.10.10} DayZ Minecraft Unloaded->Errored

See #3