presence Root object

Hello,

I have a question regarding the presence root object.

In the documentation (http://docs.alfresco.com/5.0/references/API-JS-rootscoped.html), the following is stated:
presence | org.alfresco.repo.jscript.Presence | Root object providing access to methods for detecting online presence of users.

Well we're using the Alfresco SDK 2.1, but when I use the presence.getDetails(people.getPerson("admin")) method I just get a null|null string back.

I think I'm just using the object the wrong way.

Could maybe someone explain me the use of the presence root object and what it actually does.

Thanks