Bukkit

What steps will reproduce the problem?
1.when start server, plugin doesn't load; errors appear in server.log.
2.
3.

What is the expected output? What do you see instead?

What version of the product are you using?
i tried all 1.6.2 compatible versions

Do you have an error log of what happened?
yes:

Please provide any additional information below.
server.log says:
2013-09-28 18:13:24 [SEVERE] Could not load 'plugins/LegendaryWeapons (2).jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: me/Simonster/MinecraftPlugin/MCPlugin : 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_6_R2.CraftServer.loadPlugins(CraftServer.java:239)
    at org.bukkit.craftbukkit.v1_6_R2.CraftServer.<init>(CraftServer.java:217)
    at net.minecraft.server.v1_6_R2.PlayerList.<init>(PlayerList.java:56)
    at net.minecraft.server.v1_6_R2.DedicatedPlayerList.<init>(SourceFile:11)
    at net.minecraft.server.v1_6_R2.DedicatedServer.init(DedicatedServer.java:106)
    at net.minecraft.server.v1_6_R2.MinecraftServer.run(MinecraftServer.java:391)
    at net.minecraft.server.v1_6_R2.ThreadServerApplication.run(SourceFile:582)
Caused by: java.lang.UnsupportedClassVersionError: me/Simonster/MinecraftPlugin/MCPlugin : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    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:321)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
    at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:334)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
    ... 9 more