Dan78, I can tell you where to get them: When you log on, your game automatically starts downloading them from a Sporecast called "The Robot Chicken Adventures" (or similar).
If it doesn't, there might be something wrong with your game/user/something. I don't know what, you should ask in the technical support, but as far as I know EA, at least this time, did what they said. They brought some... Allow me to say awesome, adventures when GA came out.
Wait, Dan78, on another thread on the forum I just found this link where you can subscribe to them so they will download as the Sporecasts next time you log on Spore GA:
Link
I don't know if it will work, but I guess it will.