How to import the full ITIS database (.sql) to PGADMIN 4

Question

So I need to import the ITIS database into pdadmin4 1.5 (postgresql)

The read.me file gives me these information:

1. Open a terminal or command prompt and navigate to the folder where you
   unzipped the ITIS download file.

2. Enter the following:

      psql -U root -f ITIS.sql

   If your PostgreSql user is not root, substitute your user
   name for root. Also, if you are updating a remote 
   server, you will need to  add a -h <servername> to 
   the command

3. When prompted for your PostgreSql user's password, enter
   it and the load process will start.

The directory of the file is:

/Users/kostas/Desktop/itisPostgreSql063017/ITIS.sql

How do I open a terminal or command prompt and navigate to the folder where I unzipped the ITIS download file and how do I see if my PostgreSql user is not root?

Thanks!