To get to the third skin, click the down arrow when in MEDs skin editor. Clicking twice should bring you to the third skin. I know in the version of your terrain that I downloaded, you had the skin with the alpha channel as the first.

I usually use BMP for my skins but I'm not sure if it matters or not. You don't need a texture for the skin with the alpha channel. I usually just fill it with a color that is in my texturres for my own peace of mind.

I don't have any HMP that I could send but if you zip up your terrain and all three skins, I'll fix it up for you.

-Jeff