"OpenCpn runs and initially doesn't have access to any charts, just as you have found. It expects you to tell it where the charts are, both for windows and android. So, open OpenCpn, go to the toolbar at the top, click on the crescent wrench icon, click on the chart icon and you'll see a tab labeled "chart files". you have to have downloaded charts, if your in the USA I hope you'll use NOAA charts, and put them in a file holder on the tablet. Assuming you've already done that, click on the place where they are or click on "add directory." Then check the box that says "Scan charts and update database" and finally click on OK" or "Apply"
That should work. I do most of the downloading on the PC and then transfer the data to the tablet. Works for me."