In order to whitelist a wallet that you can withdraw funds to, you must first turn Withdrawal Whitelisting on. To do this, begin by clicking on the account name in the upper right hand corner of the page and selecting Withdrawal Whitelist from the drop down menu.
In the upper left hand corner of the Withdrawal Whitelist page you will see a button to enable your Withdrawal Whitelist. Note that if you have 2FA enabled, you will need a code to enable or disable Withdrawal Whitelisting. You will also need a code each time you wish to add or remove a wallet from your whitelist.
After clicking on the Enable button and entering a 2FA code if necessary, the page will change to a blank list with two buttons in the upper right.
Clicking on the Whitelist New Wallet button will bring up a dialog box that allows you to enter the type of currency the wallet contains, the wallet address, and if necessary a 2FA code.
Once the information has been entered and you click on the add button, you will get a confirmation message. Your whitelisted wallet has been added.