Can I contact the instructor?

Not directly. The discussion forum is the appropriate venue for questions.

Instructors (or Teaching Assistants) monitor the discussion forum and try to respond to the most important questions; in many cases response from other students and peers will be both adequate and faster.

However, since there are 10,000s of learners in popular courses, it is better to assume that not all questions can be answered by the instructors who run the courses. It seems to depend solely on the policy of the university delivering each course, so be sure to read through the FAQs and other pages set up within the online course.