Hi, Meredith, I know it's confusing at first
If you connect to your site via SSH or FTP (you can use webFTP for this), you'll see your files in /home/username/domain.com/
Right there is a wp-config.php file and you want to edit that to add in the line. Make sure you add it above the line that says "/* That's all, stop editing! Happy blogging. */"