I've did exactly what the download instructions said and all the mods required to run industrial craft2 v1.81 worked fine when i tested them but when i but the industrial craft2 jar file into the mods folder and ran minecraft i got this:
Mods loaded: 4
ModLoader 1.2.3
mod_MinecraftForge 1.4.0.46
mod_ModLoaderMp 1.2.3v3
mod_IC2 v1.81
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT 6593295c --------
Generated 3/18/12 11:04 AM
Minecraft: Minecraft 1.2.3
OS: Windows 7 (amd64) version 6.1
Java: 1.7.0_01, Oracle Corporation
VM: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: AMD Radeon HD 6570 version 4.1.11079 Compatibility Profile Context, ATI Technologies Inc.
java.lang.NoClassDefFoundError: forge/IEntityInteractHandler
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at mod_IC2.load(mod_IC2.java:315)
at ModLoader.init(ModLoader.java:890)
at ModLoader.addAllRenderers(ModLoader.java:186)
at aho.(aho.java:79)
at aho.(aho.java:9)
at net.minecraft.client.Minecraft.a(Minecraft.java:395)
at net.minecraft.client.Minecraft.run(Minecraft.java:732)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: forge.IEntityInteractHandler
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 19 more
--- END ERROR REPORT 1044c01a ----------</init>