What is MCPC+?
MCPC+ is a server jar that allows you to use mods and plugins together on your Minecraft server.
Do I need MCPC+?
If you have a modpack and you wish to use plugins with it then yes.
How do I install MCPC+?
Select the MCPC+ version you need in Multicraft in the Server Type Selection dropdown, hit save changes, and restart the server.

Make sure the server jar you choose matches your version of Minecraft.
Upload your desired mods and plugins to their corresponding folder via FTP. Then Restart the Server.
Plugins Version
Due to most modpacks being built around older versions of Minecraft you need to make sure you download the correct version of the plugins.
Check what version of Minecraft you Modpack is running and download the correct plugins.
Bare in mind Tekkit Classic does not require MCPC+