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

Time: 19/06/13 21:57
Description: Failed to start game

cpw.mods.fml.common.LoaderException: java.lang.ClassNotFoundException: mithion.arsmagica.AMCore
at cpw.mods.fml.common.LoadController.transition(LoadController.java:142)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:513)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:160)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:410)
at net.minecraft.client.MinecraftAppletImpl.func_71384_a(SourceFile:56)
at net.minecraft.client.Minecraft.run(Minecraft.java:732)
at java.lang.Thread.run(Thread.java:680)
Caused by: java.lang.ClassNotFoundException: mithion.arsmagica.AMCore
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)
at cpw.mods.fml.common.ModClassLoader.loadClass(ModClassLoader.java:56)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:249)
at cpw.mods.fml.common.FMLModContainer.constructMod(FMLModContainer.java:425)
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:503)
... 5 more
Caused by: java.lang.NullPointerException
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:227)
... 32 more


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

-- System Details --
Details:
Minecraft Version: 1.5.2
Operating System: Mac OS X (x86_64) version 10.8.4
Java Version: 1.6.0_45, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 509440520 bytes (485 MB) / 581435392 bytes (554 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 and Forge are installed
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v7.51 FML v5.2.2.684 Minecraft Forge 7.8.0.684 4 mods loaded, 4 mods active
mcp{7.44} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed
FML{5.2.2.684} [Forge Mod Loader] (coremods) Unloaded->Constructed
Forge{7.8.0.684} [Minecraft Forge] (coremods) Unloaded->Constructed
ArsMagica{5.52.013} [Ars Magica] (minecraft.jar) Unloaded->Errored
LWJGL: 2.4.2
OpenGL: Intel HD Graphics 4000 OpenGL Engine GL version 2.1 INTEL-8.12.47, Intel Inc.
Is Modded: Definitely; Client brand changed to 'fml,forge'
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~~ERROR~~ NullPointerException: null

It looks like you havn't installed the mod properly. It says you havn't unloaded the Arc's mod properly. Try reinstalling it.

I've even reinstalled minecraft and everything, but I just can't because it crashes.

Quote from danymirgo

I've even reinstalled minecraft and everything, but I just can't because it crashes.

The log says that there is a missing class, which means you didn't install the mod correctly

Don't unzip ars magica in your minecraft.jar. try copying the ars magica archive into the mods folder in "~/Library/Application Support/minecraft"
Odds are you installed forge after ars magica or something, so better avoid copying mods (other than forge) into the jar at all. it'll also keep ars magica contained for easy management.