Where are you based / sailing ?
has a built in plugin for down loading free charts, but I think you have to enable it. (And it is only really great for US / Brazil
and a couple of others).
Options => Plugins => Chart Downloader 1.2 => Enable
Then when you click on the "Charts" function you will find a new tab for chart-downloader.
Options => Charts => Chart Downloader
Click "add" and select the free charts you want to download then update. It will appear in the box below (I think). Then click "download selected chart" and finally "apply".
The charts will magically appear on your world map.