The WL-138g is yet another of the Broadcom chipset wireless cards that
cause so much hassle still. Have you tried connecting via cable to the
router and checking for Hardware Drivers and also getting all current
Ubuntu updates ? The few times I've come across Broadcom wireless
recently I've found this has all that's been needed.

It might even be worth investigating changing the Wi-Fi card for one
that *is* Ubuntu friendly - they aren't exactly expensive these days
and can save a lot of hassle / hair loss Depends on how easy
this ASUS is in regard to access to the card !!