For your Nikon DSLR to work with the DIYPhotoBits.com Camera Control script it needs to be in PTP mode, not in USB mass storage mode.
- D700 : Only supports PTP mode so nothing to change
- D300: Setup Menu / USB / MTP/PTP
- D90 : Not necessary as D90 is always in PTP mode, it doesn’t support USB mass storage mode
Here is a video I made that shows you how to do it.