You top it up like a mobile mate. When it connects you can go on the Three website even if you have no credit. And top up there. Or, in a shop.

The dongle thing was 50 quid. Then I topped up ten quid which gives 1GB. That lasts a month so if you only use 800MB, then 200 is gone.... And you have to top up again.

Works for me though, as i only use it at sea.