Mob Alternatives are for changing the nationality of the enemies/mobs you fight.
To have a new countries soldiers attacking you, follow these simple steps.

1. Decide what you want to use. America, Germany, Japanese, Russians, ETC...
2. Enter the desired folder and copy the files inside, should contain skeleton/zombie & zombie_pigman
3. copy the contents of the folder. Right click, copy.
4. follow this path assets/minecraft/textures/entity
5. Paste the files into the entity folder. It should ask if you wish to replace three files.
6. Enter the game and refresh the resource pack.
7. Defeat your foes!

If something is wrong, reflect on the above steps to the letter.

Have fun!