You are encouraged to contact a tutor when you need help with an assignment. The course instructors are always happy to help students with assignments; however, they simply don't have enough time to handle all of the student questions themselves for such a large course -- that is precisely why we hire tutors. Naturally, if a tutor is unavailable, or if you feel that the instructor's point of view is required, the instructors will be happy to help.
You should first check the Announcements Page or Assignment FAQ to see if there are any known errors or problems with the assignment.
Tutors hold consulting hours in the Computer Science Consulting Centre located in MC 4065. The schedule of hours is here.
Email addresses are listed: here (for staff) and here (for tutors). Usually, course staff check their email several times a day (excluding weekends!), so you should expect a reasonably quick reply to your message.
Do Your Part! Don't expect any course staff member to hand you an answer on a silver platter! Before approaching course staff for assistance, be sure you have done your part. That generally means attending all lectures, doing the Lab exercises, and doing some initial thinking regarding assignment problems. Course staff will ask to see whatever work you have done on the problem (UML designs, pseudocode, notes from Class, etc.) before offering any assistance in debugging code.