The original poster is not using a chumby - he's using a Freescale reference board called the "MX21LiteKit". We used a similar board during our early prototype development.
Having the USB driver alone isn't enough - you also need the associated services that monitor the USB devices and perform the appropriate actions when they are attached and detached. In many Linux systems, this is done by a system called "hotplug" - on the chumby, this is done by a tool called "mountmon".
In the case of USB mass storage devices, you need to determine what filesystem it's running, load the appropriate filesystem driver, create the mount point, and then mount the filesystem. We do a little more than that - we also differentiate between certain types of mass storage devices. For instance, an iPod mounts at a different mount point, and an additional iPod-specific daemon is launched.