Here is a link to the NOAA website for Chart downloads.
Nautical Charts & Pubs
I would start with the RNC type charts, they seem to be easier to use. Once you open up the RNC page, select chart region 13 (yours) and download it.
Once downloaded unzip the folder. When unzip is complete follow Scimitar instructions above.
Here is a link to the OPENCPN instructions:
Cruisers & Sailing Forums - Announcements in Forum : OpenCPN
Alot of good info there....