Importing Google Web Fonts in headers

Hello, I am trying to change the headers on my site to Rochester. I have attempted the Google Web Fonts plug in, and inputing the html code in the Custom CSS page. Neither are working. I have a small amount of knowledge on coding, but need a little direction as to where to input the code. The .css file says not to edit it as all changes will be lost when updating the theme. Can someone help me?

Thanks
Erin