I'm looking at the help topic: "Can I get reminders or alarms for tasks that are due soon?" It says:
Yes, you can. On the Alarms page in the Tools section you can specify an email address, SMS address, Twitter account, iPhone, iPad or iPod touch that will get reminders for tasks that are due soon. You can pick which tasks get reminders by clicking the "Alarm" checkbox above the calendar when editing the task's due date.
However, I don't have a checkbox. I have a link that says "Setup Customizable Alarms/Reminders" (which I've done in the Tools section), but I can't seem to set reminders for specific tasks.