If you like to contact your Instructor as you are working on an exercise problem, you can use the “message window” to send a message to your instructor. To open the message window click on "the message tab" icon on the far right of the page as shown in the figure below.
If you like to contact your instructor after you submit your solution, you can use the "More" button located at the bottom of your solution. Just left click on the "More" button and select "Discuss with instructor".
A message window will open on the right side of the page where you can write your question and then hit "send".
Note that even though this is an instant messaging system, you may not receive an answer immediately. It will depend on the availability of your instructor. Please discuss that with your instructor to find out the best times to contact him/her if you like to receive a quicker response.