The checkmark icons should display immediately once you've added the feature. If they aren't displaying:

1. Confirm that Remember The Milk is listed under Calendars in the left column (if not, it may not have added successfully).
2. Make sure that the calendar is checked, and try unchecking and then rechecking the box.

If the icons still aren't displaying, it may be a temporary problem with Google Calendar, so you may need to check again later.

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