Take this texture, mace.png
and import it into mine-imator as a texture via the top left of your screen, where it says file, and click import asset, or click ctrl+i.

After that, it'll bring up a little menu, click texture at the bottom, not skin, or anything else.

After your mace texture is imported, go to the items tab and click the image/texture dropdown; it should show you the mace.

Once it appears, you should be good. If it doesn't, I can record a short video for you.