Activating the Pack in Your Settings
Because TZP Plus Mutants is a resource pack designed specifically for the Mutant Beasts mod, it does not turn on automatically when you launch the game. You must activate it through the vanilla resource pack menu while running both Mutant Beasts and OptiFine.
1. Pause the game or start from the title screen, then open Options and select Resource Packs.
2. Find TZP Plus Mutants in the left-hand Available column.
3. Hover over the pack icon and click the arrow to move it into the right-hand Selected column.
Setting the Correct Load Order
Minecraft loads resource packs from bottom to top, meaning packs higher in the list override those beneath them. If you use other visual overhauls or mod-support texture packs, they might overwrite the giant undead monster models if placed higher.
1. In the Selected column, use the up arrow on TZP Plus Mutants until it sits near the very top of your active list, above general texture packs.
2. Click Done to trigger a resource reload.
3. Once the game finishes reloading, standard green mutant zombies in your world will immediately switch to the new rotten skin and altered clothing textures.