I suggest that you go to www.nwa.com and register and obtain a NWA Worldperks number if you don't already have one). Then you can register for their online service, choosing your password. Once you have logged on you can go to the right top and click on "Worldperks" and then on the left of the next screen "redeem miles". then "Award Travel Reservations" and put in your desired itinerary. Make sure to first just check for IAH-AMS and no other cities as the trick is to check "Nonstop flights only". If the dates you asked for are not available, they will then give you the option to check 3 days before or 3 days after (an option that the KLM site does not give you)...

Once you have some flights that are available with the mileage that you are looking for, then look to make sure that the flight is a KLM flight and then you can put instead of AMS any city that you actually want to use as your destination. At least then you will have something to tell to Globalpass when making your request.