If you have PM's or any other notifications a floating box will appear in the lower left of the screen. It only shows up if you have unread PM's,Likes,ect.
To open whatever unread notification you have, Click on the white colored text that appears in the floating box. Example; if you had one unread "Like/s" that need viewed, the text in your floating box would look similar to what is shown here below. In this case you just click on the white text words "# 1 New Likes Received".
New Notification(s)
# 1 New Likes Received
You must view your notification(s) before this box will disappear.