Move the downloaded .fmf file to:
\Documents\Sports Interactive\Football Manager 2016\editor data\
Create folder "editor data" if it doesn't exist already.
Run Football Manager 2016 and start new save with this database enabled.
It's highly recommended to disable "Do not Add Key Staff" and enable "Add Players to Playable Teams" in advanced options.
It's also recommended to load all players from the UK & Ireland region using the Advanced... Database Size options.