Hello Roadhammer Gaming! First of all thanks for buying Tiled! Unfortunately you're running into a regression in the latest release that I'll try to fix in a 1.0.2 release soon. Thanks to your detailed report it occurred to me that I forgot to list external tilesets that have not been added to the map yet in the Terrains view. As a workaround, you can place a tile from the tileset manually first or choose Map -> Add External Tileset from the menu to add your tileset to the map.