http://blog.irrashai.com/blog/2009/03/how-to-add-static-route-in-mac-os-x/
Note: Edit the IP addresses listed below to match your network.
Create two files: /Library/StartupItems/AddRoutes/AddRoutes
#!/bin/sh
# Set static routing tables
. /etc/rc.common
StartService ()
{
  ConsoleMessage "Adding Static Routing Table"
  route -n add -net 10.0.255.0/24 10.1.1.1
}
StopService ()
{
return 0
}
StopService ()
{
return 0
}
RestartService ()
{
return 0
}
RunService "$1"
And: /Library/StartupItems/AddRoutes/StartupParameters.plist
{
Description = "Add static routing tables";
Provides = ("AddRoutes");
Requires = ("Network");
OrderPreference = "None";
}
Set everything as executable:
chmod 755 /Library/StartupItems/AddRoutes/*
Finally, reboot and check if your new routes "stuck":
netstat -rn
