I must have hit a dozen places before I found one, and it takes a battery I can only find in specialty shops.

Your old unit should have a little rectangular box inside with 8 tiny switches just like the kind you turn off your house lights with (except they are TINY!). This is the 8 bit binary code (256 possible positions with 8 toggles) for your door, depending on whether each is on or off. Set your new opener switches the same way as in your old one, and the door should open. If that doesn't work, set them all OPPOSITE the way they are in your old one (you can't tell which toggle position is off and which is on).

Good luck.