need some help seting up a forum

ok i am going to use this website: Yap365.com
they say this:

Q: Can I use my own domain name?
A: Yes, you can use your own domain if you do not want to use our free sub domain names. To setup your domain, please sign up a forum with our sub domains first; then let us know your domain name. You need set A record of your domain as: 72.9.150.116

i am using 123-reg for my domain and i wanted to know how i can set a record on my domain as 72.9.150.116

they do the rest
please help
thanks