I'm afraid I no longer use Windows Live Messenger 2009 nor the Plus! version that works with it. I vaguely remember that there is an option in the Plus! Preferences menu where you have the option to Enable or Disable certain tags like bb and html. For the [c] tags you will probably need to enable bb. Have you checked this?
Unless someone has a better solution, you may just want to try to reinstall both.
If you cannot find the installer for WLM 2009 anymore, it's available on the following site:
http://www.filehippo.com/download_msn_messenger/7560/
/ 
