The kegg application will let you know when the kegg will need to be charged. If the kegg battery is too low to use kegg, it will flash yellow when you try to turn it on. To charge, simply plug the USB cord into the charging cradle, and plug the supplied charger block into the wall. place the kegg in its cradle, and let it charge for at least 4 hours. kegg will slowly flash yellow when charging and flash green when fully charged.
The kegg might need a gentle push downwards for it to start charging like so: