How can i import a new terrain heighmap

Sorry to keep bugging but searching gave me nothing on this apart from the note in the docs thats say --
"this allows you to import a heightmap", well i guessed that much

ive created my heighmap but when i select General > Bitmap in the terrain editor nothing appears in the list and no way to navigate to another folder.

does the heightmap need to be in a certain folder? if so which? or does it need adding as a resource like when i add a new texture? if so which .cs file?

Thanks