Where is usr/local?

Where should it be? By default when you open the file manager you run around your home directory(/home/username), same as in windows C:\Users\Username . Notice how /usr/local starts with backslash ? There's root directory from which you can access other directories; the image to keep in mind is the root directory is where root starts, and everything else is branching form it

How to access the folder you need:

Method #1 : press CtrlL in the file manager ( which is called nautilus, by the way) and type /usr/local into the address bar or / .

Method#2: Pressing repeatedly AltArrow Up buttown in file manager will get you to root directory (which is like C:\ folder in windows), and navigate graphically to whatever folder you need from there.

Method #3: Press AltF2 to open "Run command" dialogue and type nautilus /usr/local

cd /usr/local

will get you there

You do need root privileges to edit files there, in which case you can invoke sudo, be very cautious about meddling with files however in the /usr directory and in the root directory / in general.

BTW don't be afraid of the command line! A useful introduction to the CLI is available here.