You don't go anywhere on WikiTree to view the message. You will have received the entire message by email. WikiTree just takes a message someone writes and sends it to your email address (without showing your email address to the person who wrote the message).
You can send a reply from your email program, just like you would to any other email message you get. Please note, though, that when you reply, your email address will be visible to the person you send it to.