First I added myself to vboxusers group:

# grep vbox /etc/group
vboxusers:x:128:
# usermod -G vboxusers -a von

In order to get this group change to take effect in my login session, I had to logout and back in again.

Then I edited /etc/fstab to add /proc/bus/usb. I had an old entry I had to comment out.

# vi /etc/fstab
# tail -2 /etc/fstab
#/dev/bus/usb /proc/bus/usb usbfs auto 0 0
none /proc/bus/usb usbfs devgid=128,devmod=644 0 0

I had to unmount the old /proc/bus/usb and then remount it:

# umount /proc/bus/usb
# mount -a

At this point I restarted VirtualBox and was able to mount USB devices.