We have the demo version. Tried open it up, but it said Minecraft has crashed.

      Minecraft has crashed!      
      ----------------------      

Minecraft has stopped running because it encountered a problem; Exception in server tick loop



--- BEGIN ERROR REPORT d6bed04f --------
Generated 10/28/12 2:41 PM

- Minecraft Version: 1.4.2
- Operating System: Mac OS X (x86_64) version 10.7.5
- Java Version: 1.6.0_37, Apple Inc.
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
- Memory: 501528840 bytes (478 MB) / 554369024 bytes (528 MB) up to 1069416448 bytes (1019 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
- Is Modded: Probably not
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
- Player Count: 0 / 8; []
- World crative ts SUPERFLAT Entities: 0 total; []
- World crative ts SUPERFLAT Players: 0 total; []
- World crative ts SUPERFLAT Chunk Stats: ServerChunkCache: 0 Drop: 0
- Type: Integrated Server

java.lang.VerifyError: (class: ih, method: c signature: (III)V) Bad type in putfield/putstatic
	at ge.a(SourceFile:216)
	at bce.b(SourceFile:66)
	at net.minecraft.server.MinecraftServer.r(SourceFile:731)
	at net.minecraft.server.MinecraftServer.q(SourceFile:661)
	at bcb.q(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:595)
	at fp.run(SourceFile:818)
--- END ERROR REPORT 4938005d ----------