Bukkit

When I try to launch the server, I receive this error message. I'm using v 0.5.0 on my mac osx 10.8.2 with Java 7. From what I can tell the problem has something to do with the difference between Java 6 and 7, but I couldn't get the plugin to work after trying either.

02:17:13 [SEVERE] Could not load 'plugins/Sewers.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: me/hammale/Sewer/Sewer : Unsupported major.minor version 51.0
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:184)
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
    at org.bukkit.craftbukkit.v1_5_R3.CraftServer.loadPlugins(CraftServer.java:239)
    at org.bukkit.craftbukkit.v1_5_R3.CraftServer.<init>(CraftServer.java:217)
    at net.minecraft.server.v1_5_R3.PlayerList.<init>(PlayerList.java:55)
    at net.minecraft.server.v1_5_R3.DedicatedPlayerList.<init>(SourceFile:11)
    at net.minecraft.server.v1_5_R3.DedicatedServer.init(DedicatedServer.java:106)
    at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:382)
    at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
Caused by: java.lang.UnsupportedClassVersionError: me/hammale/Sewer/Sewer : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader.java:80)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:53)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:247)
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
    ... 9 more

Hmmm, I have only seen this error when Java 7 code runs on Java 6. Make sure you are running Java 6