Tell users to enable it as a block

Newbie users may get confused as there is no reference to having to enable the instant messages as a block.

Perhaps an enhanced description:

The Instant Messages are sent via a messaging block that allows you to select the user and then send him/her/it a message.  The message appears at the top of the users page.

...and in the Install or Readme file, a reference made to activating and nominating the position of the Instant Messaging Block via admin->blocks

It is much better than chatbox, thankyou.