Bukkit

Hello!

21:39:08 [GRAVE] Could not load 'plugins\ServerMessage2_v_1.1.jar' in folder 'pl
ugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError
: com/github/seplugins/servermessage/ServerMessage : Unsupported major.minor ver
sion 51.0
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava: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.ja
va:239)
        at org.bukkit.craftbukkit.v1_5_R3.CraftServer.<init>(CraftServer.java:21
7)
        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.jav
a:106)
        at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java
:382)
        at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:5
73)
Caused by: java.lang.UnsupportedClassVersionError: com/github/seplugins/serverme
ssage/ServerMessage : Unsupported major.minor version 51.0
        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 org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader
.java:80)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.
java:53)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava:173)
        ... 9 more

I use groupmanager and Craftbukkit 1.5.2 Beta

I don't understand the error. Can you help me?