Hi there,
You need to use this firmware for the WRT54G up to V4:
http://www.hotspotsystem.com/firmware/openwrt/LINKSYS/openwrt-wrt54g-squashfs.bin
If it only accepts trx files then this one:
http://www.hotspotsystem.com/firmware/openwrt/LINKSYS/openwrt-brcm-2.4-squashfs.trx
Steve