I'm using bbpress 0.73?
Go to line 48 and change add_action('bb_init', 'online_update'); to add_action('init', 'online_update');. Then it should work.
I'm getting MySQL errors?
You'll maybe get an error if you browse your forum the first time after you've installed the plugin. This is normal.
If you keep getting errors, try setting $mysql41 to false maybe it works then, otherwise don't hestitate to contact me.
Where is the online-count for my statistics?
This feature has been removed at the moment, sorry.