Did you try the newer version? You cannot export the addresses from the newer version of blockchain.info wallet. Why do you want to do so anyway? If you want to use it on a new wallet, try transferring the funds by sending to the address on your new wallet.
I think it is available if those addresses are created or imported to blockchain.info in past. There is option to view private key of every addresses you have added under setting > my imported addresses.
But for only blockchain given bitcoin address there is no option to get private key and this address keep changing also. Better send that bitcoin from your wallet to new address created as paper wallet or try creating new address in blockchain.info.