You can reply to the messages in 2 ways:
1) From your account:
Click on the "Messages" in the top right hand corner and choose View messages from the bottom of the pop up window.
Once in the messages list, click on the message you want to reply to. As you open the message, you will see the message thread. At the bottom of the screen you will see a "quick reply" option or just type in your message and press "send".
2) From email notifications:
When you receive a request from the member or a simple reply, you will receive an email notification to your email. You can also reply from this email as well.
Open the email and scroll down to see their message. Just below the message, you will see orange "reply" button. By clicking on that button, you will be redirected to your account and to that message.
If you are not logged in to HomeExchange.com on that device, you will be prompted to Sign In first.