If you are using this menu option in previous versions of sharepoint, there is no link for "Sign in as Different User" anymore...
But there is 2 workarounds for having the same functionality;
1. Change the siteurl with yours and it will ask you for the new username and password
2. Open internet explorer by using "run as different user" option (Shift+ right click) by entering the username and password that you like
Keep in mind first option may not work in some browsers and ie10