Relax, it really is not as bad as some of the posters paint the picture here.
Yes, incidents do happen and that is inconvenient.
But it is absolutely not the case that all seat requests are ignored or that all seats obtained via OLCI are given away to someone else as a rule of thumb.
Some posters tend to focus only on the instances that go wrong unfortunately.

For US stations handling is done by NWA, so you need to go to nwa.com for OLCI.
This facility opens up 24 hours before departure (30 hours for non-US flights on klm.com).
Once you check-in for your AMS-bound flight, you can usually also check-in for connecting flights even though connecting flights are >24 before departure.

Getting a "good" seat like exit row, bulkhead etc depends on
1) your time
2) if the station pre-assigns these seats before check-in opens
3) load

Some stations will pre-assign all the Elite passengers to decent seats.
KLM allows pre-seating via Reservations/travel agents up to 80% of the capacity in Y-clss. But only for intercontinental stretches. For intra-Europe stretches you can only obtain seats upon check-in.

See seatguru for very good advice too.
Have a good trip!