https://www.stellar.org/blog/bitcoin-claim-lumens-2/

It could make for a good day for bitcoin on the 26th but given everything going on with the hard fork/soft fork situation it's hard to say anything with certainty. Any news on that would probably have a more dramatic effect than the giveaway, but don't take my word for it!

It looks like you'll need access to your private keys so I don't know if you'll be able to claim Lumens from Bitcoin on an exchange unless the exchange has already prepared for the giveaway.