1. 	If you've used DNSenabler, or if you're using Mac OS X Server, or if you have in any other way activated Mac OS X's built-in DHCP and/or DNS servers, disable them.  This would usually involve checking that they are either set to -NO- or absent altogether in /etc/hostconfig.  If you've never done anything to do with DNS or DHCP servers on a client version of MacOS X, you won't need to worry about this; it will already be configured for you.

2.	Add a configuration item to /etc/hostconfig as follows:

DNSMASQ=-YES-

3. 	Create a system-wide StartupItems directory for dnsmasq:

sudo mkdir -p /Library/StartupItems/DNSmasq

4.	Copy the files DNSmasq and StartupParameters.plist into this directory, and make sure the former is executable:

sudo cp DNSmasq StartupParameters.plist /Library/StartupItems/DNSmasq
sudo chmod 755 /Library/StartupItems/DNSmasq/DNSmasq

5.	Start the service:

sudo /Library/StartupItems/DNSmasq/DNSmasq start

That should be all...