These are open air festivals as far as I know and I pretty much doubt they will have a mail box set up for fanmail. Under Impressium http://www.rock-im-park.com/impressum you can find addresses and email addresses, I guess the best thing is to send an email and ask.

Hi

I am planing to write to Jay-z, but i don´t know if he signs VV. I also don´t know if the Venue are accept Fanmails! Can someone help me??

There is 2 venues he gonna sing:

6/4/10: Rock am Ring, Germany [germany]
6/5/10: Rock im Park, Germany [germany]

It is a festival. I don´t know the address. Can someone help me with that?

http://www.rock-im-park.com/index.html
http://www.rock-am-ring.com/index.html

Their official websites