Miniportal (functions.php)

I used the miniportal to make some changes to my site, such that i'd like to intergrate the forum with mys ite.
In the steps in the portal Step 2: add to functions.php  am i supposed to create a neww functions .php file or it already exists and i have to add.
I f it exists i cannot locate it.In which folder is it in?

Thank you for your continued Support.Cheers.