Not able to specify access by site for differnt users (no replies)

Hello,

I'm newbie and searched for this question on forums database but not found anything.
I've installed my piwik on my server and looks great, I have several sites and I've created the sites on my admin panel.
I need to have different users access only to one of the sites.

In this tutorial : [piwik.org] there is a selector for to choose the website and specify the user. But in my piwik installation this selector list is not shown (the captured screen is attached)

How can I specify which user can read the data por each site?

Thanks a lot and kind regards