You need to move the open end of the posts closer together so you can pop that plate into place. To do that, bend the chain sideways at that master link to get it to close.
There's a bit of technique, but it's not too hard to close (opening it again often requires three hands - the extra one to lift the plate off. Sometimes the plate will pop off if you bend the chain a bit further, sometimes not).
1. Move the slotted plate so it's over the post ready to close.
2. Hold the chain with one hand each side of the master link with the slotted plate towards you.
3. Pull your hands apart slightly so that the chain is straight.
4. Now bend the chain so the master link moves away from you (and the open end of those posts get closer together).
It might take a couple of attempts but it can be done (the chain-bending is the same way I push chain into chain tubes on my recumbents, when I can't life the recumbent to get the tube vertical)