Are you following all of the directions in the Creating Skins from Artisteer Templates? Also, you mention Artisteer 3.x, but the minor version is actually really important in this process. The 3.0 and 3.1 versions have different instructions. I'd recommend 3.1 if you can get that, since the process to convert Artisteer exports to mojoPortal skins is much simpler than it was for older versions.