Auto-Activate Plugin on User Blog?

I think I am confused about how plugins work under WPMU. I thought that putting plugins in the mu-plugins folder meant that they didn't have to be activated by the user. I am using the bm-custom-login plugin to show a branded login screen on my user blogs, but putting that plugin into the mu-plugins folder doesn't seem to make it automatically activate for new users. It only seems to work when I put it in the regular plugins folder.

What I have been doing is going into every new blog account and activating it by hand, but that obviously can't be a solution for long. Any idea how I can get this plugin to auto-activate? And again, I apologize if this question is just beyond stupid.

thanks