You need to go into war3x.mpq and extract TerrainData.slk. Change the neccessary field in there (I forget the name, but its pretty obvious), and import it into your map. Voila, buildable terrain.
cool i dident knew that u could do it in other ways.. now u can have more titels in 1 insted of like tile1 buildable, title1 unbuildable.. thanks.. :D :D
This is the correct file. I've commented each column so you know what it does. Be careful, messing with certain parts of the .SLK could cause Warcraft to crash.
Just import it into your map and set the path as TerrainArt\Terrain.slk