allora ho provato ad installare la gem mod per 1.2.4 ho installato il modloader di risugami e fin qui tutto ok, avevo anche tolto il META-INF inserisco il file della gem mod che nelle istruzioni non chiedeva altro ed ecco quello che mi dice minecraft 
Mods loaded: 2 
ModLoader 1.2.4 
More Gems Mod v2.5 
      Minecraft has crashed!       
      ----------------------       
Minecraft has stopped running because it encountered a problem. 
--- BEGIN ERROR REPORT 9ffd2338 -------- 
Generated 03/04/12 19.53 
Minecraft: Minecraft 1.2.4 
OS: Windows XP (x86) version 5.1 
Java: 1.6.0_30, Sun Microsystems Inc. 
VM: Java HotSpot Client VM (mixed mode), Sun Microsystems Inc. 
LWJGL: 2.4.2 
OpenGL: GeForce 7600 GT/PCI/SSE2 version 2.1.2, NVIDIA Corporation 
java.util.zip.ZipException: invalid entry CRC (expected 0x0 but got 0x4b597e83) 
at java.util.zip.ZipInputStream.read(Unknown Source) 
at java.util.zip.ZipInputStream.closeEntry(Unknown Source) 
at java.util.zip.ZipInputStream.getNextEntry(Unknown Source) 
at ModLoader.readFromClassPath(ModLoader.java:1204) 
at ModLoader.init(ModLoader.java:886) 
at ModLoader.addAllRenderers(ModLoader.java:189) 
at ahu.<init>(ahu.java:79) 
at ahu.<clinit>(ahu.java:9) 
at net.minecraft.client.Minecraft.a(SourceFile:267) 
at net.minecraft.client.Minecraft.run(SourceFile:650) 
at java.lang.Thread.run(Unknown Source) 
--- END ERROR REPORT a16ed3c2 ---------- 
qualcuno mi può spiegare dove ho sbagliato?