Just add the jar to your project like you added Bukkit, and make sure the library is included at runtime.
All you need to do is Project>Properties>JavaBuildPath>AddExternalJars then click the API then click OK
Well, if you're using methods from an external plugin then that plugin will have to be in your /plugins/ directory.
getServer().getPluginManager().getPlugin("API") != null Checks if the plugin manager have a plugin named "API", it will return null if it doesn't!