Bukkit

What steps will reproduce the problem?
1.Starting the server

What is the expected output? What do you see instead?
For it to work

What version of the product are you using?
Latest

Do you have an error log of what happened?
Yep
Could not load 'plugins\RealisticChat.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/pl/MYCRAFTisbest/RealisticChat/RealisticChat : 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: com/pl/MYCRAFTisbest/RealisticChat/RealisticChat : 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.java:173)
    ... 9 more

Please provide any additional information below.
I am using Iridium Server Manager to run CraftBukkit, and I have 30 somthin other plugins that work perfectly fine.