When iRemindU is launched, it will check to see if it has to remind you of something. If it does, a window will appear and will be on top of every other window currently open on your computer.
This comes in handy because what good is a reminder that's behind Safari? To set a reminder, you will need to go to File > New Reminder. This will bring up a new window where you will see a text area to type in, a disabled checkbox and button.
Now you need to do is type in your message, so pick something really important, that if doesn't get done, you could be in a lot of trouble...
After that, you should have your reminder all ready to go, press the Save button to save it so that it's there the next time you launch iRemindU. Finally press the now enabled "Activate" checkbox to activate the reminder process.
Now, the next time you launch iRemindU a little reminder will float above your computer to remind you of whatever it is you need to do, in this case type the iRemindU Manual.