It took me a while to figure this one out, because I didnt have to do anything!
i wanted my reminders from my reminders.app in my iphone and ipad to be in sync with each other and with my mac.
i thought they might go into mail.app, or ical.
turns out they are in the reminders section of ical.
you just need to let ical sync with your icloud account (and do the same on your ios5 devices)