what to do?
Rename your wallet.dat file to something else, fire up the new wallet again so it generates a new wallet.dat file, copy the new DRK address, close the wallet, rename the wallet.dat to something else, rename the first one back to wallet.dat, fire up the wallet again, send your balance to the new address, close the wallet, rename old wallet.dat again, rename the new one back to wallet.dat, fire up the wallet again, all is well.
Faster to do then to type!
edit: or do what innergy said, that will let you keep the existing address.
Thank you innegy and thelonecrouton, but i'm asking why i downloaded a wrong wallet from the official download page? i'm not an expert so i ask you: can i send now my money to my exchange and then cancel this wallet?
You didn't download a wrong wallet from the main site. Things changed from RC3 to RC4 to RC5 and for safety sake, it's smart to send all your coins from old wallet.dat files to a newly generated wallet in the latest build or dump your private keys and import into the latest wallet if you care about the address associated.