I don't know if the Guilds card is similar, but the Dark Ages card says to go to www.playdominion.com/dark. On the other side there are instructions about swiping the card and at the bottom there is a circle that says "Hold Here", below that there is a 6 digit number (maybe your card doesn't have one?). I just went to the url, clicked through some stuff, logged in, put in the code and it worked. I did this a while ago, so I don't remember all the details. Try going to playdominion.com/dark instead of "guild" the guild one seems to just redirect straight to the normal Dominion online home screen.
edit: The playdominion.com/dark page has a link which sends you here: http://play.goko.com/Dominion/gameClient.html?boxcard=true. At the bottom left corner of the page there is a little icon that says Redeem.
The Guilds card has
www.playdominion.com/guild [sic] which just redirects to the regular gameclient page, without the boxcard true part and thus without the redeem part.
I used your 'dark' link and got to the part where I can type in my 6 digit code. Every time I get "Failed to read card Please try again".
Good job, Goko. Good job.
The good news is that my Cool Stuff Inc order came today and I got Base Cards in it.
The bad news is that my Base Cards promo does not have a 6 digit number. [edit: but it has a different URL, which tells me to use 200001 as the code].
I now have Haven, Hoard, and Nobles. Nothing else though.
...and I just clicked Play Bots to try to test out the cards and Goko wouldn't start a game for me. Well, at least this has been more fun than work
Edit: tried again, got an all base bot game, and lost (oh god the shame) and then a base+nobles bot game.
Thanks again for everybody's help with the mystery