first you have to compile your proxy with ACL support.
./configure --enable-delay-pools --enable-linux-netfilter --enable-arp-acl --disable-ident-lookups --enable-snmp --enable-removal-policies
( in upper configuration line --enable-arp-acl enables your squid proxy to support MAC address access list)
use following command in your access list to create access list with mac address.
acl maclist arp "/usr/local/squid/etc/maclist.txt"
(in upper line "maclist" is the name of access list and arp is for mention that this access list is for MAC addresses , and in qoutes "/usr/local/squid/etc/maclist.txt" , maclist.txt is the file which contain your mac addresses, on mac address per line.)
Hope this will help you.