youritem/blocknamehere.blockIndexInTexture = ModLoader.addOverride("/terrain.png", "/mymodfolder");
and make a texture 16x16 pixels
in the spot "/mymodfolder", place a folder in your bin/minecraft folder and then add the navigation to the /mymodfolder
1+ me if I helped (the green button in the bottom right conner of this post)
9
I'm running mc 1.0.0 on a mac.