Click Show Options
on the far right above the messages or click the down arrow 
to display the option buttons.
Using course tools>>Discussion Board>>Participating in forums
You can mark selected messages as read or unread in a Discussion Board forum. The titles of unread messages appear in bold.
Click Show Options
on the far right above the messages or click the down arrow to display the option buttons.
Do one of the following:
Select each message you want to mark as read
or unread by clicking the box 
to the left of it.
Select all messages by clicking Select All.
Click either Read or Unread.