Only admin users have access to all rosters. If you're a teacher, you will need to ask an admin or the teacher that created the roster to share the roster with you.
STEPS 3 & 4
Click "My Rosters" and select "All Rosters"
Click the "Share this roster" button
STEPS 6 & 7
Choose Yourself and Any Other Admins or Teachers to Share the Roster with and Save