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I wanted to install forge for Minecraft on a clean .jar file.
I used the default Ubuntu Archive Manager to put the files into the jar and then launched it.
After loading the Mojang logo, it crashed and showed this report:

---- Minecraft Crash Report ----
// Sorry 
Time: 2/14/13 8:14 PM
Description: Failed to start game
cpw.mods.fml.common.LoaderException: java.lang.ClassNotFoundException: tdwp_ftw.biomesop.mod_BiomesOPlenty
at cpw.mods.fml.common.LoadController.transition(LoadController.java:130)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:504)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:161)
at net.minecraft.client.Minecraft.a(Minecraft.java:410)
at asq.a(SourceFile:56)
at net.minecraft.client.Minecraft.run(Minecraft.java:744)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.ClassNotFoundException: tdwp_ftw.biomesop.mod_BiomesOPlenty
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:185)
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:57)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at cpw.mods.fml.common.FMLModContainer.constructMod(FMLModContainer.java:418)
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:69)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:317)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:300)
at com.google.common.eventbus.EventBus.post(EventBus.java:268)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:153)
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:69)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:317)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:300)
at com.google.common.eventbus.EventBus.post(EventBus.java:268)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:86)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:494)
... 5 more
Caused by: java.lang.NullPointerException
at org.objectweb.asm.ClassReader.<init>(Unknown Source)
at net.minecraftforge.transformers.EventTransformer.transform(EventTransformer.java:29)
at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:228)
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:173)
... 32 more

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------
-- System Details --
Details:
Minecraft Version: 1.4.7
Operating System: Linux (amd64) version 3.5.0-22-generic
Java Version: 1.6.0_38, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 1649157040 bytes (1572 MB) / 2058027008 bytes (1962 MB) up to 4772200448 bytes (4551 MB)
JVM Flags: 2 total; -Xmx5120M -Xms2048M
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 v7.26a FML v4.7.32.553 Minecraft Forge 6.6.1.524 4 mods loaded, 4 mods active
mcp [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed
FML [Forge Mod Loader] (coremods) Unloaded->Constructed
Forge [Minecraft Forge] (coremods) Unloaded->Constructed
BiomesOPlenty [Biomes O' Plenty] (tdwp_ftw) Unloaded->Errored
LWJGL: 2.4.2
OpenGL: Mesa DRI Intel(R) Sandybridge Mobile GL version 3.0 Mesa 9.0, Intel Open Source Technology Center
Is Modded: Definitely; Client brand changed to 'forge,fml'
Type: Client (map_client.txt)
Texture Pack: Sphax PureBDCraft 128x MC14.zip
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~~ERROR~~ NullPointerException: null

I know that in the crash it says "Caused by: java.lang.ClassNotFoundException: tdwp_ftw.biomesop.mod_BiomesOPlenty" because I tried installing the BiomesOPlenty mod which requires Forge. Is there a problem with installation, compatibility, system, etc. ?

You sure that you didn't had any mods folder? : tdwp_ftw.biomesop.mod_BiomesOPlenty