As others have said, unscrew the battery door, and the instructions are on the back of the cover. Always start with batteries removed, then install fresh batteries. On mine, you press the 3/4 button, and the "Key" button, It beeps, you then enter your code and hit the "Key" button again. If it does not work the first time, try manually locking the door, then trying again.
I have found that when the batteries start getting low on voltage, the unlocking/locking will be inconsistent. If you are getting that behavior, you need to change the batteries. We just got back from an 11 week trip where we used the locks multiple times a day, and I found that my batteries lasted about 6 weeks. If you use the rig on an occasional basis they should last much longer, but I recommend lithium batteries as they will hold a charge longer and are not as sensitive to temp swings.
We did have to have the original unit replaced under warranty, but the replacement has been very handy, especially with kids coming and going w/o keys. That said, I have a realtor style hide-a-key hanging in the engine compartment with an extra key so I never get locked out.
2016 Seneca 37HJ
Toad - 2011 Rubicon Unlimited (Built to crawl)