There's many reasons why your Craftbukkit Plugin may not be loading, check out the following suggestions:

• Ensure the plugin is Compatible with your Craftbukkit version. Go to the Bukkit Plugin page, and check this.
• Check for any pre-requisite (required) plugins, that your plugin needs to run with. (Also on the Bukkit Plugin page)
• Try re-uploading/installing the plugin, sometimes it may become corrupt during the upload.
• Your plugin may need to be configured before use, or have specific installation instructions. Read the Craftbukkit Plugin page, looking at the installation instructions.
• If you've changed the plugin's configuration files, you may have configured it wrong.
  • Use a file editor such as notepad++ to ensure you correctly configure the file text format. 
  • If you still can't get it working, delete the configuration file, and let it regenerate.