Ok so firstly take off the lock.
It is quite confusing.
the metal needs to be fitted on the UNDERNEATH door. so the one that slides under the other one. Not the one on top.
Open the doors so the top door is open all the way and not covering the other door. (as in push the underneath door to one end then pull the top door over to complete opposite side so you have access to the right hand edge of the underneath door)
Unscrew all the screws and orientate the metal strip so that the long edge is on the outer part of the door.
As in, hook the small part round the side of the underneath door and put the screws back in to tighten it up.
Then close the top door again (the metal will be in the middle, if you've closed the doors and it's on the side, it's wrong!)
With the long metal arm on the outside of the underneath door you can now fit the lock.
It is hard to work out the orientation without the instructions.
Typically the underneath door is the left hand door.
Hope this helps...