You mean blockchain.info right ? Open the website
www.blockchain.info from your computer.Login to your account ,there is an option to import the private key.
yes you need to access blockchain.info via their web wallet as exporting feature is not available for android or iso app. And its not import buttom but the way there is export buttom.
Thank you all of you and particularly OP as i was also having this doubt and today i got cleared that export feature is for importing Private Key and through Mobile we cannot use this feature. Now my problem is solved and i have downloaded my private Key.