The Freecom FSG-3 firmware user interface does not have the ability to set static IP addresses for hosts via DHCP. Luckily, the underlying dnsmasq program does this (and lots more) easily, and even though dnsmasq is in /usr/local/sbin for some reason on the FSG-3, it still reads the default configuration file at /etc/dnsmasq.conf (which Freecom does not use, so the file does not exist by default, and therefore does not get overwritten by the default firmware).
To set static IP addresses for your hosts, just add lines like the following to /etc/dnsmasq.conf:
dhcp-host=00:04:5a:0f:b2:44,nslu2,192.168.1.6,infinite