HELP: How do I import/use custom terrain in a TFT map?

[Fell_Paladin]

I have created a custom ground texture, imported it into a map, but whenever I try to change the path to the exsisting art I want to replace it changes the file to the exsisting file in the .mpq. Example:

DIRT! TerrainArt\BlackCitadel\Citadel_Dirt.blp

Magically becomes the file:
Citadel_Dirt TerrainArt\BlackCitadel\Citadel_Dirt.blp

This is really annoying because I REALLY need my custom terrain for the map I am making. Can anyone give me a crash course in uploading and using custom terrain art and cliff art? Especially with the "Replace Terrain" command involved. Thanks a ton.