Responsive.css not active in child theme

Hello! I have created a responsive.css in my child theme (just copy-paste - not sure if I need to add an @import line). I understood from a thread in the kathmandu theme that I need to add some code to the child function.php file for it to work, but with the kathmandu example in hand, I cannot seem to figure it out. Can you help me do that? Thanks a million! Sandra