DMZ for VOIP telephone adapter

I would greatly appreciate if someone could explain how to proceed.

I am setting up my Grandstream VOIP adapter behind my airport extreme. (I previously had it behind the cable modem (Optimumonline) and in front of the Airport but i could not get Back to My Mac to work.)

I called the VOIP company (ViaTalk) and they suggested putting the Grandstream into the DMZ. They were kind enough to send instructions but they proved to be useless as they are for a generic router in a windows set up.

The only information I was able to find on the web are the following instructions;

"I am using one of the new Airport Express base stations with my ViaTalk and it works great. They key is, under the NAT tab, do TWO things.

ONE: Give the ViaTalk adapter a static DHCP setting by using the Mac address and telling the Airport to always give your ViaTalk TA the same IP.

Then, make the IP of your viatalk adapter the DEFAULT HOST. This is known on most routers as the DMZ. This will open all ports to your TA and voice quality will be outstanding so long as you're not saturating your internet pipe. I have a 15mbps/2mbps connection so that's not really a problem for me."

Is this good advice? It is from a two year old post. If it is I don't know how to assign the "mac address to the particular device in step 1.

If this is bad advice could someone please advise me how to add the device to the dmz?

Thanks