Buildcraft/Modloader rapport d'erreur

Bonjour,

En démarrant minecraft j'ai cette erreur :
java.lang.NoClassDefFoundError: BaseModMp
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
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$000(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 ModLoader.addMod(ModLoader.java:270)
at ModLoader.readFromModFolder(ModLoader.java:1293)
at ModLoader.init(ModLoader.java:826)
at ModLoader.AddAllRenderers(ModLoader.java:186)
at wb.<init>(wb.java:76)
at wb.<clinit>(wb.java:9)
at net.minecraft.client.Minecraft.a(SourceFile:265)
at net.minecraft.client.Minecraft.run(SourceFile:644)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: BaseModMp
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)
... 20 more
Comment réglé ce problème ? Merci d'avance.