Activating Plugins in mu-plugins folder

Sorry to be such a newb, but I am just learning and have lots of stupid questions. One is about plugins. I'll use the blog activity plugin as an example,

The install instructions say
1) Place the files in the specified directories
2) Visit the plugin page under the site admin - the auto-installer will attempt to install the plugin
3) If the auto-install does not add the db table(s) for you then run the sql code in "sql.txt" on your wpmu db

I don't quite understand the sentence at number two. I thought that plugins only show up on the plugin page when dropped into the wp-content/plugins. I thought that when you drop them into wp-content/mu-plugins they were activated site wide (via the options plugins check).

When I go to the plugins page i can not see the blogactivity as an option to activate.

Can you please tell me what I'm doing wrong or missing?