after the registration the plug in show the account page of the user, is it possibile to show something else waiting for the admin approval?
I'm sorry but there is no way to change redirection.
As per user approval the registration is being processed through default WordPress registration system. If you already have a registration approval function in WP then it'll work for wpForo. If you don't have such function please search for Registration activation plugins, they should work for wpForo too. These are two good examples:
i use WP-approve-user, and i have the forum locked for the not logged user, can i do something to show some messagge to the user of the waiting time ?
This message should come from the plugin. I'm sorry, but wpForo doesn't have an option for this.