Supporter Plugin for Multi DB

When following the instructions on https://premium.wpmudev.org/project/supporter/installation/ for installing supporter, it reads:

If you are using Multi-DB:

* You need to add the global table lines to db-config.php BEFORE installing the Supporter plugin or running the sql.txt
* Add these lines to your db-config.php if using multi-db:

add_global_table('supporters');
add_global_table('supporter_signup_stats');
add_global_table('supporter_daily_stats');
add_global_table('supporter_ads');

* If the auto-install does not add/modify the db table(s) for you once you've installed the Supporter plugin then run the sql code in "upgrade_sql.txt" on your wpmu db

I do not know where to locate the db-config.php file. Please let me know.