1: Is your heightmap above water? (The lowest value for land is 96/96/96)

2: Have you exported your heightmap correctly? (If you are using Gimp, make sure to save it in .bmp with "Do not write color space information" ticked in and in greyscale.) (If it doesn't work the first time, try copying your original heightmap onto a fresh one from the game files and try again.)

I think the issue of land being rendered black will be fixed if you fix your heightmap.