In your squid.conf add this at the end of the acl section
auth_param basic program /lib/squid3/squid_ldap_auth -R -b "dc=domain,dc=com" -D "cn=ldapuserwithaccess,cn=Users,dc=domain,dc=com" -w "theuserspassword" -f sAMAccountName=%s -h XXX.XXX.XXX.XXX
auth_param basic children 5
auth_param basic credentialsttl 5 minutes
acl ldap_users proxy_auth REQUIRED
where XXX should of course be replaced by the ip of your ldap server.
Add this at the top of the http_access section of squid.conf
http_access allow ldap_users
Might be that your squid_ldap_auth is located in another directory depending on how you installed it.
The above works perfect for me on my reverse proxy using Squid 3.
Don't forget to restart Squid with (this one is for Debian, don't know yours)