USB mass storage is handled differently with the RC30 update than it was with previous versions. With RC30 you won't automatically see the "enable USB for mass storage" in your SD card and phone storage settings beforehand. Instead, you will receive a notice in the notification bar after you plug in your USB cord to your PC.
Try unplugging your USB cord, reboot your phone, then plug in the USB cord again. You should receive a notice in the notification bar offering to mount the SD card. You need to activate mass storage AFTER it's plugged in by clicking on that notification.
Good luck!