These are the lines you want to add/modify in your squid.conf file :

#defining an external ACL.
external_acl_type nufw %SRC %SRCPORT /usr/local/bin/squid_nufw_helper -f /usr/local/etc/squid_nufw_helper.conf

#define the actual ACL
acl auth_nufw external nufw

#And at the place wanted in your config :
http_access allow auth_nufw

#Dont forget the order matters !!