You can ask a follow-up question by commenting on his/her answer: click "add a comment" right below the answer. Alternatively, message them directly via the italki messaging system: click on their profile picture to go to their profile, then click "message" on the right, in the green box.

It is necessary to click on their FOLLOW menu (in brown color) so that you can message a person privately if that is what you wish. Otherwise, use the "comment" feature.