In the system there are 224 national teams - all members of FIFA and the 14 team without full membership.
If you want to play with a weaker national team you have in the beginning, when selecting databases, to select the Advanced tab and select "all players with nationality".
Step 1Extract the downloaded file using 7-Zip for Windows or The Unarchiver for Mac.
Step 2Move the extracted .fmf file FIFA Nations League by FM-Players.net.dbc to your editor data folder:
Documents > Sports Interactive > Football Manager 2015 > editor data
Important: Create the "editor data" folder if it doesn't exist.
Notice: There may be two "Sports Interactive" folders in your Documents library. If this is the case then put it in the one which has your saved games inside.
Step 3Start a new career game (won't work with classic mode).
A little pop-up window will come up that says there is X enabled editor files. Select "Custom" from the editor data drop down menu and choose the databases you wish to load in your new save.