How to use Google+ with Google Apps account
For enabling Google+, you need to first activate 'Picasa Web Albums' and 'Google Talk' service for your domain. If these services are not enabled then Google+ will not automatically become available, even if your domain has chosen to automatically enable new services.
Log in to your Google Apps administrator account and go to 'Organization & users' tab.
In the left sidebar choose the organizational unit and then click on 'Services' tab. Scroll down to find Google+.
Click on 'ON' button in front of Google+.
At last 'Save Changes' that you have done.
After you have enabled Google+ for you domain, each user of your domain has to individually sign up for Google+ profile at google.com/+.