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WoW Model Viewer, open it up, go to Item\ObjectComponents\Weapon\ in the tree list, and find your mace model. Then rename the m2 you downloaded to the name of the mace model (will be "mace_1h_blahblahsomething.m2" or "mace_2h_blahblahsomething.m2"). Then, add it to a new MPQ, making sure to keep the filestructure, and add the Model.blp to the root of the MPQ (not inside any folders). Put the MPQ in your \Data\ folder, put Cypher's MEFix in the same folder as WoW.exe, and run the MEFix.