I tend to think she already knew of the existence of PRISM its all just pretending just like she pretended how furious she was when she learned that the NSA is tapping her phone.
And we all know that her government is in bed with the usa gov
http://rt.com/news/us-drones-germany-ramstein-625/
You are right, madame Merkel is pretending to not know. No idea why you Germans still vote for her Truth is, Germany is sending personal information of German citizens over to the States 24 hours a day, 7 days a week. In turn, the German secret service gets access to the PRISM user interface. Don't worry about it, you have nothing to hide, they only kill you based on this data