WordPress.org

For standard WordPress:

The plugin should be placed in your /wp-content/plugins/ directory, so it looks like this:

/wp-content/plugins/
/wp-content/plugins/wibstats/
/wp-content/plugins/wibstats/wibstats.php
/wp-content/plugins/wibstats/wibstats-includes/

For WordPress MU or BuddyPress:

The plugin should be placed in your /wp-content/mu-plugins/ directory (not /wp-content/plugins/) so it looks like this:

/wp-content/mu-plugins/
/wp-content/mu-plugins/wibstats.php
/wp-content/mu-plugins/wibstats-includes/

Wibstats for WordPress MU requires no activation. The database table for each blog should be created automatically.