Folks are telling the OP that he could use blockchain.info to do the work he needs to do (sign a message). This is true, but such advice should come with a strong warning: once you give your private key to blockchain.info you're relying on them to keep it safe---ie, you've got multiple points of failure: yourself, bc.i, network transport. Anyway, this may be fine, but if you were going to use that address to store any significant amount, I, personally, would opt for a local solution where you don't have to send your key over a network to a third party.
Agreed, I would like to stress this point too. I recommend Electrum instead of using any web wallet where you will share control of your private key.