Just a little heads up, I have been playing with this issue for a while. I could create a reminder on either my iPhone or on my iPad and it would update. However the reminder I created on my calendar would not flow to the iPhone or iPad and the reminders that I created on the iPhone or iPad did not appear on my Mac.
Open the calendar program on the Mac on
go to iCal, then slide down to preferences
look to see if the iCloud account is there if not add it and and enter your password