Setting Permissions to Install SEO Plugin

I am trying to install the SEO Ultimate plugin so that my website is mobile friendly. It is installed but not functioning due to my inability to set permissions. Below is the error I got:

Please set the permissions on this folder (/home/content/72/11873472/html/wp-content/plugins/wp-mobile-detector/cache/) to be 777 to allow the WP Mobile Detector to work properly.

Execute the following command via SSH:
chmod 777 /home/content/72/11873472/html/wp-content/plugins/wp-mobile-detector/cache/

I have looked at numerous forums and as a result I have found the index.php file to try to change my permissions, but there is nowhere for me to enter 777.

Help please!