For a step-by-step visual on setting up the manager and importing mods, check out this tutorial:

The encryption key for Frosty Mod Manager is a specific alphanumeric string required to access and modify the game's encrypted Frostbite engine data. FIFA 18 Encryption Key

A common misconception is that you can download an encryptionkey.bin file or find a script that auto-injects the key.

If you are using the demo to test mods:

You might wonder: “If FIFA 18 is years old, why is the encryption key still a problem?”