To enter a lesson, you will use the "Go To Lesson" button on your lessons page.
You can visit your lessons page here: https://www.chegg.com/tutors/lessons.
You can also enter your lesson using the link that was messaged to you from your tutor. If you are unable to enter the lesson for any reason, make sure that you're signed into the correct Chegg Tutors account.
If you are part of a parent/student joint-account, be sure that you're signed into the student account. You won't be able to enter the lesson from the parent account.