Firstly, unzip the downloaded archive and upload the included files to the relevant 
directories on your board as listed below:

Plugin/inc/plugins/board_messages.php to /inc/plugins/
Plugin/inc/languages/english/admin/board_messages.lang.php to /inc/languages/english/admin/

Now you can install & activate the board messages plugin from the plugin manager in the acp.

Remeber to setup your admin permissions once you activate the plugin.  You'll need to grant yourself access to the new Board Messages section of the Configuration area in the acp.
