Single Sign On and AD users

hi guys

all the time when I install vCenter I created a local group in my Windows vCenter Server "Virtual Adms" and I'd add all the users that needed admin access to vCenter and in vCenter I added that group to Administrators in vCenter Permissions.... also I created a local account and add that account to "Virtual Adms"

Now I am trying to do the same thing for vCenter 5.1

Right now I am only able to login to vCenter using the local account the AD accounts won't work....If I understand correctly I need to configure and SSO thing in order to be able to login my users is right?

In fact when I try to add new user in permission in my Web Cient I don't see the option for my domain? this is a screen taken form another user in this forum but is the same thing

http://screencast.com/t/lf22UBY8

is that or I am missing something else here?

thanks