If you are on Windows:
Download "Eternum_feature_mod_rpa.zip", unzip it, find where you have "Eternum.exe", there should be a folder "game" with "archive.rpa" inside. Put "mod.rpa" file from the archive you downloaded into that folder, right next to "archive.rpa"
If you are on Android:
Download and unzip "Eternum_feature_mod_rpa.zip". Put "mod.rpa" file into the "Documents/Ixalon/com.caribdis.eternum/game" folder. The folder should be in your "Internal storage" and should also contain "archive.rpa"
If you are on Mac:
Follow the instructions in README.txt, they are basically step-by-step
Make sure you have 0.7.5 version of the game (you can check it by opening the game and opening "Credits" screen) and don't have any other mods installed.
If everything is correct, you should see the phone icon below the heart icon and Achievement button in the Heart screen (the one that opens when you click the heart icon)