I just got this to work on my Snow Leopard MacBook Pro.
1. Activate the modem and install the software on the USB device using a Windows PC. I was able to do this on an WindowsXP machine in about 15 minutes. Don't forget to program the USB device (The step after receiving your account number).
2. Top off or add credit to your account and associate it with a "data plan". For example, I purchased a 20 dollar top up card from Best Buy, added the top up card pin number to my account, and then associated the funds from the top up card with the 20 dollar "data plan". The website should now report data remaining and expiration date.
3. Plug the card into the Mac. You should get a pop up with drivers and a manual. Install and follow instructions from the manual. If you have problems, try downloading the drivers from the Novatel website:
http://www.nvtl.com/index.php?option=com_content&view=article&id=163&Itemid=536
Download the driver for the USB connected MC760
4. Connects like a champ.
It actually took me quite a while to set it up because I didn't program the modem after activating it. After that, it was pretty straight forward.
If you don't program it, it will only access the virginmobileusa.com site, VERY VERY slowly.
Good luck! Hope this helps.