Check these....
Heightmap
-513x513
-Greyscale mode (Very important)
-Saved in the Terrains folder try in your maps folder
-Saved as a .png format (Also very important)
-Try saving as MAPNAMETerrains.png
Mask
-513x513
-RGB mode (Very important)
-Use ONLY pure red, pure green, pure blue
-Saved in same folder as the heightmap
-Saved as .png (Very important)
-Try saving as MAPNAMEMask.png
If you use GIMP and you ever get the message "GIMP can not handle .jpg or .png" (Or something like that) and you get the options "Export" "Ignore" and "Cancel" Choose Export then Load Defaults.