You need to install plugins whilst you are not logged into MSN, so fully exit MSN and run the installer that comes with the plugin. If it only consists of a dll, place that into the plugins folder (C:\Program Files\Messenger Plus! 3\Plugins) and restart your MSN.
If after that they still aren't in the plugins list, it's most likely they will not work with the version of MSN Messenger you are using.
P.S. edit (edit button on bottom of your post) your first post instead of doubleposting
Edit: yw
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