From CreeperHost Wiki

It's a little confusing, but while mods and plugins may look like they do the same thing on the surface, they're built to run on different versions of Minecraft. We call the normal no-extra-bells-and-whistles version of Minecraft the 'vanilla' version - this is the untouched version of Minecraft that you can download from the website. Then there's modded Minecraft; mods are extensions to the game that require a different core Minecraft .jar file to run on - Forge (Feed the Beast), ATLauncher and Technic. Plugins need the Bukkit .jar file to run. Ordinarily, you can't run mods on a Bukkit server and you can't run plugins on a Forge/Technic/ATLauncher server - they're not cross-compatible without the installation of what we call a 'bridge' - the Cauldron/MCPC+ software.

So, in order to use plugins on your server you'll need to be running software that's compatible with plugins; either Bukkit, or if you're using Forge, ATLauncher or Technic, you'll need to install MCPC+/Cauldron

For Modded Servers

Forewarning: Cauldron/MCPC+ doesn't always play nice with some mods on modded servers - ALWAYS make a backup of your server before switching to Cauldron, just in case! For this reason, we don't support the software, so please take care as our support will be limited.

If you're running a Forge, ATLauncher or Technic server, you can grab the relevant version of Cauldron from our Control Panel > Minecraft > Versions. Choose the version relevant to whichever software you're running.
Once that's installed, start your server up once and Cauldron will generate the Plugins folder, and you will be able to install plugins from the "Plugin Manger" option on the left! You should be able to install plugins from there. :)

For Bukkit

If you have Bukkit software installed, just go to your Control Panel > Minecraft > Plugin Manager and enter the name of the plugin in the search box; this will find the plugin from the Bukkit plugin repository and you can install it from there. If you don't have Bukkit software installed, you can install it from your Control Panel > Minecraft > Versions.