If you'd like to check if the feature has rolled out to your account, make sure to have the latest version of Instagram installed. Pop open the the app and tap the Options icon in the top right of your Profile page. The bottom of this page should now show an 'Add Account' option, if the feature has been enabled for you.
As we mentioned before, the feature is currently rolling out slowly. It could be a few days before all Instagram account have gotten the feature enabled.