Originally Posted by linglingfool

If you absolutely have to buy it at the airport, and you want someone who is likely to speak some English, go to the o2 store. For €25, you can get 5GB of high speed data (throttled thereafter), and calls back to the US are €0.29/min. If you're willing to wait until you get into town, you can cut that to €15 and .09/min, but the discount providers don't have a big presence at the airports.

Walk into the o2 store and tell them you want an "o2 Loop Freikarte" (not a "Smart" Freikarte, that requires a monthly commitment) with €60 of "guthaben" (credit). That will give you ~10 minutes a day for 2 weeks. You will have to give them your passport with your details, but they can activate the SIM for you. Once that's complete, ask them to activate the "Internet-Monats-Flat L" for you. If they can't do it for whatever reason, send "START INTERNETL" to 5667, and respond "ja INTERNETL" to the confirmation SMS you'll receive. Afterwards, it might take an hour or two to fully activate, but you should be good to go.

If you run out of credit on your trip to make calls with, walk into any grocery or electronics store and ask for o2 guthaben in the amount you desire, which usually had a €10 minimum.