First of all, what have you done so far in the way of trouble shooting?
Have you tried our SEARCH feature ? There are many questions about this so maybe that would be a good place to start?
Are you plugged into a USB port or a HUB? BBs do not like HUBs so plug into the USB on the laptop.
Are you using the cable that came with the BB?
ARe you using the Mac version of DM or are you using Bootcamp or similar on a Windows instance?