Go to system setting or something similar. Check the firmware by going to settings->Info and look at the firmware number. If it 01.xx.xx then its V1 and rockboxable(is that a word???) If it starts with 03 instead of 01 then its not able to run rockbox but should be able to read SDHC cards anyways.
Next make sure its in MSC mode in the Settings->USB settings or something similar to USB settings(can't remember but im sure you can find it.
If its V1 and its still not rockboxing then I would try an older rockbox cause I couldn't get 3.1 which was just released to work so I went with 3.0 and its worked like a charm since.
If it still doesn't seem to work then maybe its a raphsody model or something. I heard those can't run rockbox.