To ensure you actually make the mask correctly, try double checking wiff dis tut
http://www.youtube.com/watch?v=tN6lMKdKBkQ&list=PLCrTZGtLFJkJeB1qYGae-WGR_gSEj_xPf&index=2Another thing that I want to point out since I'm not sure how you did this after reading the post if you actually make it correctly.
When you save your terrainmask, make sure you actually overwrite the old copy of it that didn't look as planned, but also you "MUST" restart the game in order for the terrainmask changes to take place, otherwise it will just keep showing the old one.
And just some info ontop of that.
Another thing is that if you for some reason like to check the map inside the game with your character and not only in the map maker, is that the exported map works as 1 map, the one you edit in the map maker is another, which means if you export a map you will have 2 copies of the same map in cape until you remove the exported one, meaning that changes you make to the one in the map maker will
not show on the exported one, and you must check which one of these portals that is the one in your map folder for the map maker.
Non the less, if you have issues with either, just check on the tutorial linked at top and it will explain what you need to know in order to get a terrainmask into your map.