I think, at least for that route, the Thalys website links into
b-europe.com for actual booking - so not surprising you're finding the same problem in both.
I'm still not clear what route you're trying to book, but it sounds like:
11:41 IC2010 Brussels-Nord, arr Antwerp Centraal 12:22 (platform 21)
12:30 Thalys 9327 Antwerp Centraal (platform 22), arr Amsterdam Centraal 13:42
I
think platform 21 to 22 at Antwerp is just a cross-platform change - but if not, they're certainly both on the same (lowest) level, so it'd just be up one escalator, across the tracks, and down one escalator.
This would be ticketed as a Thalys ticket Antwerp-Amsterdam, plus a 'any Belgian station' add-on.
However, as
stut has said, you'd be better off taking the Thalys straight through from Brussels South station.
I'm not sure why your credit card is not being accepted.
NSinternational.nl (and Dutch websites generally) tend to charge more if you can't pay direct from a Dutch bank account - but I've had no problems using non-Dutch/non-Belgian credit cards on both
NSHispeed.nl and
b-europe.com in the past.
EDIT: I see you've now managed to book the direct Thalys - I'm pleased that worked out. Yes, you should be able to take a train from Brussels North to Brussels South to connect with your Thalys. Journey takes 7 minutes, and there are trains at least every 10 minutes at that time of day. I
think your Thalys ticket includes train travel from Brussels North to Brussels South (see '
Extend Your Journey Free of Charge' on the Thalys website) but perhaps someone else can confirm this.