Composition Not Enabling After Installation Due to Unsupported Java Version

Purpose

The following stack trace can be seen in the logs:

2014-11-19 10:29:10,578 ERROR [TP-Processor19] 
[atlassian.plugin.manager.DefaultPluginManager] 
enableConfiguredPluginModule There was an error loading the descriptor 
'composition-setup' of plugin 
'net.customware.confluence.plugin.composition'. Disabling.
-- referer: http://xtest.wiki.www.uprr.com/confluence/plugins/servlet/upm 
| url: 
/confluence/rest/plugins/1.0/net.customware.confluence.plugin.composition-key 
| userName: iosg004
java.lang.UnsupportedClassVersionError: 
net/customware/confluence/plugin/composition/macro/CompositionSetupMacro : 
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 
org.apache.felix.framework.ModuleImpl$ModuleClassLoader.findClass(ModuleImpl.java:1829)
at 
org.apache.felix.framework.ModuleImpl.findClassOrResourceByDelegation(ModuleImpl.java:716)
at 
org.apache.felix.framework.ModuleImpl.access$200(ModuleImpl.java:73)
at 
org.apache.felix.framework.ModuleImpl$ModuleClassLoader.loadClass(ModuleImpl.java:1690)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
...

Answer

The error you received most probably relates to the Java version that your instance is running on. We built our add-ons in accordance with the current supported Java platform by Atlassian. More information can be read here: Supported Platforms

First, check the Java version your instance is running on:
java -version
If the version you're running is lesser than what is recommended by Atlassian in the Supported Platforms page, upgrade your Java to the advised version.