You can download it and install manually from http://code.google.com/p/wl500g/downloads/list.
The below script is for those who are not too familiar with Linux however e.g. mounting share from NAS requires inserting some module not in the firmware.
I assume here that you use Raas images e.g. from here.
Note: this is for firmwares from google project, not for earlier versions.
Code:
KVER=$(/bin/uname -r)
DIRM=/opt/lib/modules/${KVER}
FVER=$(cat /.version)
case "${KVER}" in 2.6*)
CPUV=$(sed -n '/.*MIPS.*74K.*/p' /proc/cpuinfo)
if [ -n "${CPUV}" ]; then MVER=-MIPS32r2; else MVER=-MIPS32r1; fi
;;esac
cd /mnt && rm -f modules.tar.gz && wget -O modules.tar.gz http://wl500g.googlecode.com/files/modules-${FVER}${MVER}.tgz
MDIR=/opt/lib/modules/${KVER}_${FVER##*-rtn-}
mkdir -p ${MDIR}
tar xvzf modules.tar.gz ./lib/modules/${KVER}/
cd ./lib/modules/${KVER}/ && cp -r * ${MDIR}
rm -rf ${DIRM}
ln -s ${MDIR} ${DIRM}
lsmod
Note: You have to run the above script after every firmware upgrade as module version has to match firmware version!