Have you tried using firestarter: http://www.fs-security.com/
or you could add under su permission to access the port you are using for squid (default 3128)
Depending on your firewall rules this will vary somthing that looks like:
iptables -A INBOUND -p tcp --dport 3128 -j ACCEPT
This allow you to control access via acl in squid.conf
Or you could use a command like this
iptables -A INBOUND -p tcp -m tcp -xxx.xxx.xxx.xxx --dport 3128 -j ACCEPT
For each ip address or use the appropriate netmask /24 for all 192.168.0.0 to .255.
In the examples above INBOUND is the rule set which maybe different on your system something like
RH-Firewall-1-INPUT for FC4 out of the box so try the following:
iptables -A RH-Firewall-1-INPUT -p tcp -m tcp -192.168.100.100 --dport 3128 -j ACCEPT
and try and access the proxy via the machine 192.168.100.100 unless that is the server then change the numbers you get the picture.
One last thing if you su then unless you su -l you'll wnat /sbin/iptables