My Mc crashes when I try to join a server or open a world.

Help everytime I open minecraft and try to run a world Minecraft crashes . I recently tried installing singleplayer commands. I have so many great worlds but I don't want to delete them. . here is the error:
Minecraft has crashed!
----------------------

Minecraft has stopped running because it encountered a problem; Updating screen events

A full error report has been saved to /Users/nonibouche/Library/Application Support/minecraft/crash-reports/crash-2013-04-09_19.41.04-client.txt - Please include a copy of that file (Not this screen!) if you report this crash to anyone; without it, they will not be able to help fix the crash



--- BEGIN ERROR REPORT a57ef381 --------
Full report at:
/Users/nonibouche/Library/Application Support/minecraft/crash-reports/crash-2013-04-09_19.41.04-client.txt
Please show that file to Mojang, NOT just this screen!

Generated 4/9/13 7:41 PM

-- Head --
Stacktrace:
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 java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:96)
at bjh.<init>(SourceFile:35)
at net.minecraft.client.Minecraft.a(SourceFile:1378)
at axu.e(SourceFile:166)
at axu.a(SourceFile:131)
at axr.a(SourceFile:72)
at axr.d(SourceFile:127)
at axr.m(SourceFile:111)

-- Affected screen --
Details:
Screen name: bkg
Stacktrace:
at net.minecraft.client.Minecraft.l(SourceFile:1078)
at net.minecraft.client.Minecraft.K(SourceFile:574)
at net.minecraft.client.Minecraft.run(SourceFile:526)
at java.lang.Thread.run(Thread.java:680)

-- System Details --
Details:
Minecraft Version: 1.5.1
Operating System: Mac OS X (i386) version 10.6.8
Java Version: 1.6.0_43, Apple Inc.
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Apple Inc.
Memory: 479931280 bytes (457 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 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: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
LWJGL: 2.4.2
OpenGL: ATI Radeon HD 2600 PRO OpenGL Engine GL version 2.1 ATI-1.6.36, ATI Technologies Inc.
Is Modded: Very likely; Jar signature invalidated
Type: Client (map_client.txt)
Texture Pack: Clear Glass, Green Grass
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~~ERROR~~ NullPointerException: null

java.lang.IncompatibleClassChangeError: Implementing class
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 java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:96)
at bjh.<init>(SourceFile:35)
at net.minecraft.client.Minecraft.a(SourceFile:1378)
at axu.e(SourceFile:166)
at axu.a(SourceFile:131)
at axr.a(SourceFile:72)
at axr.d(SourceFile:127)
at axr.m(SourceFile:111)
at net.minecraft.client.Minecraft.l(SourceFile:1078)
at net.minecraft.client.Minecraft.K(SourceFile:574)
at net.minecraft.client.Minecraft.run(SourceFile:526)
at java.lang.Thread.run(Thread.java:680)
--- END ERROR REPORT 7d18310 ----------