There is a pinswitch on either the door or the doorjamb. Find the switch and press it, see if you can arm it then. If you can't, you can either get a new switch, make sure the old one is actually connected, or bypass the switch altogether. If you want to bypass it, you should be able to just disconnect it. I think the switches complete the circuit when the door is open, I don't remember though.
If it works when you manually press it, the switch might need to be adjusted, or the hatch might not be lined up properly. The dome light should work like normal if the switch is working..