To enable task reminders in Remember the Milk, turn on the option under Settings > Reminders and choose your method(s). RTM supports reminders via instant messaging (AIM, GaduGadu, Google Talk, ICQ, Yahoo! Messenger, MSN Messenger, Jabber, and Skype), email, and TXT messages. Add as many options as you wish.
Make sure RTM knows your correct time zone (Settings > General), or reminders will arrive at the wrong times.