Well, read the message ( Re^5: file not found with OPEN ), when you do, there will be a link that says "comment on" or a link that says "reply", click that link, and you made a reply directly to that message

The alternative is using the chatterbox,  /msg [Laurent_R] thanks for your help

Regarding thank you nodes, its ok to summarize :) What do you think about "Thank you"-nodes?