Amazing disappearing plugins

Hi,
Not sure what the heck I have managed to do now. I seem to have installed the mysterious Harry Potter invisibility addon. Plugins are vanishing as they are activated?

For example: In Network Admin I had installed members directory and blogs directory. If I activate these on a site or network activate them from admin they then can not be found in the plugins list on the site. They still show up in network admin and of course FTP but nowhere at all on a site, even a plugins search returned no results. Crazy