arp change in start up script

Our networking chaps changed the way in which my xserve connects to the world.
My xserve 10.4.3 sits in a firewall and used to connect to a router and all was well, they changed it to MPLS and a switch and my server cannot get out to the world. The ip address is the same it seems that it cannot resolve the mac address to that ip. Now if I open up terminal and do an arp -s ipaddress, mac address then all is well.
Not being a unix guru, I need to make this permanent. I have been told that I have to edit rc.local, but when I look on my system I do not have one?
Could some kind soul make me a scrpit with any ip and mac and tell me exactly where to put it (be kind).
The reason being is that I work in a windows centric environment and as usual I get stick etc etc.
cheers
Mike