there isn't any way to reset the locking system on this van. The reason the alarm horn is sounding and the doors won't lock is that the central locking control module (BCMii) is detecting that a door either hasn't locked or that a door is registering as still open.
Make sure that the side door is closed properly and that it is aligned correctly, incorrect alignment is normally down to a collapsed center door roller, if this is the case the electrical contact pins on the door miss the contacts on the body of the van causing a locking failure or a door ajar signal even with the door closed.
Check that the wiring isn't damaged in the drivers side rear door as this can lead to a door ajar signal. Try locking all the doors manually using the actuators, use a screwdriver to set the rear door latch to closed while leaving the door open, this way if the rear door won't unlock you can still access the latch assembly to unlock it manually , if they all lock see if any won't unlock this may point you to the correct door.
If you can't see anything obvious then unfortunately a live data tool needs using to pinpoint which door is giving the incorrect signal to the BCMii, Ford IDS would be best for this.
If you take the van to your local dealer there are 2 outstanding safety recalls on this van, these will be done free of charge, the Ford reference numbers are 7C108 and 8S187, one is for the fuel pipes and one is for a check on the front suspension.
Let me know if you require further assistance and I'll reply as soon as I'm able to.