I have got a green exclamation mark next to two of my domains, and it said it's comment box, but I can't open it to see the comments, I know one of them had an offer.
The green exclamation mark shows up if you have set a comment in your front office for your own use. If you roll over it, it should show you the comment you entered.
If you received an offer on a domain, you can log into your account, click NDX Market on the left and then click Active domain sales on the left.
You can see a video on how to respond to offers here.