You need to be on the Dawnguard side. And you cannot ask her about a cure until AFTER the main quest is over. If you ask her about a cure during the main quest she will say it is a gift, not a curse, and she will never agree to cure herself. After the MQ, if you never asked her about a cure, then you can now ask and she will disappear for a few days and come back cured.

Still,its cool. But would be weird to have a non-vampire serana.