To reply to a ticket, go to the Tickets admin screen. Click on the ticket which you want to reply to. (Alternatively, you can click the link in your email notification which you received when the customer opened/replied to a ticket.)
The is the “Edit Ticket” page, and this is where you (support agents) reply to tickets. The ticket content is at the top, and all replies are listed underneath:
If you want to see a list of all tickets by this customer, you can click “(customer tickets)” in the Stakeholders meta box. That will show a list of all tickets for the customer, like this:
On the “Tickets by Customer” screen, click “Go Back” to go back to the ticket. After you type a reply, click either “Reply” or “Reply & Close” at the bottom (“Reply & Close” will simultaneously reply and close the ticket).
The customer will get an email notification of your reply (you can disable notifications in the plugin settings).