We had already reported that Google has made its new Social Networking site Google+ available foe all, even its released for Google Apps users, but if your Administrator has to activate it first and then only other user will be able to use it.
So if you are administrator then here is the way to enable it :-
Login to your Google Cpanel, your cpanel will be at :-
https://www.google.com/a/cpanel/domain, (domain is your domain registered for Google App)
Now move to Organization & Users and then scroll down and you will find Google Plus option,
Just turn it ON,
Then you will see a message, Just click on Turn Google+ On and save changes,