Minor technical info: The Giveaway wrapper uses Internet Explorer settings to connect to GAOTD's server. If the settings under your Connection tab under Internet Options isn't correct, you'll have trouble activating. Also, as always, check your firewall. It may be set to block the Giveaway wrapper.
w8tif's question: Why not just give us the serials? Well, it's mostly a matter of choice for the developers. They may want to keep everyone in the dark so that they can't upgrade to the next version (although the conditions of GAOTD are no free upgrades, some developers don't create a special GAOTD version of their software with a special key to prevent upgrades). Also, it may be because that they don't want to overload their activation servers, if applicable. However, mainly the reason is that they don't want people to know their serial numbers.