At AMS, you can use one of the desks by the machines. Elsewhere, you just go to check-in, and you keep your pre-assigned seat.

I don't understand why they limit it to the places with the machines - it's a nice benefit to be able to use OLCI even without them. You end up with daft situations that, for example, when you make connecting flights through AMS, you can OLCI the 2nd section, even though you're going to be picking up the BP from an airport without a machine. If you can do this, they might as well let you do the whole lot...