$ yaourt -S isatapd
to install the package.
The package add an rc.d script /etc/rc.d/isatapd to run isatapd as a system service. You need to modify it to set your router. Change the value of ISATAPD_ROUTER to the hostname of the router you want to use.
If you want to modify other options instead of using the default values, have a look at the project's homepage for further reference.
After you have done with the configuration, you can add the script to DAEMONS in /etc/rc.conf to have it start when you boot the system. Make sure to have it go after your network configuration scripts. It's recommended to have it start in background (with an @ in front) to save your boot time.