The Ez-Flash tool does a decent job at detecting optical drives. I've found the easiest way is to burn the new ROM file onto a CD (re-writables work well) and it should pick it up fine.
EDIT: As previous posters have mentioend, NEVER EVER use the Asus Windows updater. Many stories of it going wrong on the Asus forums alone. It nuked my Striker Extreme BIOS and I had to hot-flash to recover, something I never really want to attempt again.