How to make plugins to be on 'plugins' folder to work from 'mu-plugins' folder?


I installed several plugins to my WPMU 2.8.4 installation and some plugins were specifically for MU, and among the others some work from the "mu-plugins" folder even when they are supposed to be at the "plugins" folder, some plugins does not even show on the list, any reason?

Please advice, look forward for a solution

Thanks & Regards