Keith, I understood your question, and here's a simple answer:
The first (main) disconnect (breaker, in this case) is exactly where the neutral and ground are to be separated. The incoming cable contains two hots and (typically) a bare: the service-cable neutral.
However, the outgoing cable should contain two hots, an insulated neutral, and a separate bare; the equipment grounding conductor. This is because the neutral and ground must be separated beyond the main disconnect.
So, all three conductors (the incoming bare, the outgoing bare, and outgoing insulated neutral) should be connected to this neutral/grounding terminal block. Be sure that the enclosure is bonded to this block, too.
Sometimes, the block is bolted directly to the enclosure already, and sometimes you must install a screw or jumper, like in a main-breaker panel. The interior breaker panels do not require main breakers; you can use main-lug panels.
In addition, the breaker panels must NOT have this jumper or screw installed, and you may need to add grounding strips, if there are not separate neutral and ground busses already. By the way, these panels are now actually sub-panels.