email plugins don't show up after activation

I am having trouble activating email or newsletter plugins. I get the Plugin has been activated message, but the plugin pages never show up in the dashboard. I have been installing and deleting several email and newsletter plugins trying to find one that will do what I need, so I’m wondering if one of them has messed something up with my installation. I had the email users plugin installed and activated in the past, and it was working, but deactivated it so that it wouldn’t interfere with other email plugins, but now I can’t reactivate it. A couple of others that I’ve tried to re-activate are emu2, and ALO EasyMail Newsletter. I have upgraded to 3.04 yesterday, but I seemed to be able to activate these plugins after I upgraded, so I don’t think that is the issue. Any suggestions?