Help with Minecraft Bukkit plugins

So I have a new Bukkit server on mac and I have the latest version of spigot and whenever I put plugins in the plugins folder it doesn't work.

This happens: Could not load 'plugins/MassiveCore.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginExceptio… Java.lang.UnsupportedClassVersionError: com/massivecraft/massivecore/MassiveCore: Unsupported major.minor version 51.

The plugins don't even register!

I can't post the whole log so ill post some of it if you need me to.

krivleon2011 12.04.2017

Make sure the plugins are Spigot supported

Exavius 12.04.2017

"Could not load 'plugins/MassiveCore.jar' in folder 'plugins' "

MassiveCore.jar is a compressed archive. The plugin is within. Go back to the website to find the installation instructions.

SilverTonguedDevil 12.04.2017