In the upper left you will find the three lines which will bring up some options (see below).
When you are ready to send a message or video chat simply select the + New Conversation. You will then be able to type in name, email, or phone number to get started. Once selected you will then see the options to send a message (bottom right) or start a video chat (video in upper right).
Embedded below is a 2 minute and 2 second YouTube tutorial on the ins and outs of the extension.