Why am I not able to view or edit others' to-do list?

Your account is limited only to your calls/call backs. This setting has been made by the account’s primary user. Your account’s primary user can change permissions in the Sub-User Accounts section of their account.

How do I create a to-do item?

Click on “Calls/Call Backs” in the left menu and the click “Add Call” or click the in the Quick Navigation (in the upper right corner).