There's no way to get the message off without putting the sensors on. You can reset the system and get the indicator light off, but it will come back after awhile. If you want to get it back to where it just has the message on startup,
1. Depress parking brake.
2. Turn ignition to run (but dont start.)
3. Turn headlight switch from all the way off to all the way on until your horn sounds (I think it's four times.)
4. Turn off ignition and start as normal
This should take away the warning light temporarily, but if you want to get rid of the error permanently you have to install the sensors.