When someone sends a private message, if they are logged into wikitree, it will automatically include there email address and it will be on the message somewhere - at the bottom of the message This message is from ... but the sender can remove their address. If they aren't logged into wikitree, they have to add their email address and I just tested you have to put an email address in. So an email should be on the message.
Try just clicking reply to, but make sure it's not going to wikitree.