You cannot. That feature has been was "temporarily" cancelled due to numerous reports of stuck transactions and growing awareness of its privacy limitations:

https://blog.blockchain.com/2014/03/17/shared-coin-suspended-temporarily/

http://www.coindesk.com/blockchains-sharedcoin-users-can-identified-says-security-expert/

I dont think the previous version of SharedCoin is ever coming back because its GitHub page is now gone:

https://github.com/blockchain/Sharedcoin