1) First check that you have an active network connection (needed to download charts) and that you have the correct version of SeaNav for your area - i.e.. icon in launcher is titled SeaNav UK for the UK version and SeaNav US for the US version.
2) We have found a bug in the current version of SeaNav whereby if we don't load the charts when we first run, then we don't try again - its possible that is what you are seeing. To fix this, you'll need to quit and restart SeaNav - hopefully just once, but maybe a couple of times.
To kill the app, double-tap on the iPad home button, then find SeaNav in the apps list, then tap and hold its icon in that list until a "no entry sign" appears. Tap the no entry sign to "kill" the app - then restart, and we should hopefully see it begin to find and download charts.