you cant
You drag it into the plugins folder. If you want plugins in your minecraft server use bukkit, not the vanilla minecraft server.
Install bukkit
You go to BukkitDEV or Bukkit Plugin list. Then Download and take the .jar into the plugins Also, some come with folders so put the folder in the plugins folder aswell. *Note* You need a Bukkit server for this.
First you install bukket, then drag the plugin into the Plugin Folder
In WordPress.com blogs you cannot install plugins even if you pay. The custom css allows only to change the styling of themes. It doesn't allow you to modify theme code or add plugins. If you want your own plugins, install WordPress.org software in your own domain and server.
Go on bukkit plugins and get a saving program that should help it and if it doesn't work re-install the server files.
It is a server that dosn't use mods or plugins
Yes, mods are meant to be installed client side before playing on a modded server. What you may be refering to are plugins which are like mods but are made for servers only.
Download the latest CraftBukkit server from bukkit.org(top-right corner of the home page). For plugins, click on 'Get Plugins'.
You can't... To make a server that can use plugins, you setup a Bukkit server from their website... it's not possible to put plugins on the Minecraft_Server.exe type servers (not currently anyway)
You need a server using Bukkit, and there are multiple plugins for it.
Place the plugin into the plugin folder and restart the server.