WordPress.org

1. Upload 'SeoPress' to the '/wp-content/plugins/' directory
   
2. a) If you use standard WordPress installation:
   Activate the plugin through the 'Plugins' menu in WordPress
   
3. b) If you use multiple Blog installation:
   Activate the plugin by the Network Admin plugins menu
   
4. If you use Buddypress:
   
   Be sure that the main blog header title have only included wp_title() or bp_title() function.
   
   

That's it, have fun!
Plugin is updatefriendly, all needed migration from older versions, will be done on activation.