The miner uses your Private ACTIVE key for mining, according to "A Quick Guide on How to Mine Steem on Windows"

– If you are not sure where to get the private key, then just login to your steemit.com account, go to your Profile and then to the Permissions tab. Here you need to click on Show the private key for Active (you may need to relogin first). Do note that the key shown by default is the public key, you need to show the private key and use the private key in the config! Alternatively you can use the Owner private key, this is also your login password, but due to security reasons you better stick with the Active key.