I'm not here all the time to answer your questions - have to earn a living!
try this page Installing Charts | Official OpenCPN Homepage
I have never used the chart downloader. You must find and download the charts you want, then point opencpn
at the directory you copied them to.
for the USA;
For those interested in U.S. waters, raster charts and S57 vector charts are available as free downloads from NOAA. Find all NOAA charts, both raster (RNC) and vector (ENC) charts, for your region through NOAA OCS Website at: Nautical Charts & Pubs
. US charts cover a wide area, apart from the mainland states, Puerto Rico
, the US Virgin Island, the Hawaiian Island chain as far as Midway Island, American Samoa
and the Northern Marianas Islands are all included
It should be pretty straight forward.